But anyhow, I will put the rest under a cut, since I expect this to get long.
Okay, first for the series in general. The SMT games revolve around, uh, demon summoning. Man, that sounds wrong, doesn't it? But it deals with you working with demons, or summoning them briefly to do things for you, or something similar. How it works in each game is slightly different. But keep in mind that "demons" in this case is a shorthand for supernatural beings of pretty much any flavor - the more typical demons like incubi, succubi, and named devils, mythological beasts, gods, spirits... etc etc.
Generally you can form some sort of pact with some of the demons that you meet, and then can later on fuse the demons you've been working with to form new, more powerful demons, while continuing to make pacts (or whatever) with more. These beings are generally used in combat, although some of the games have other uses for them as well.
Combat tends to be turn-based, making solid use of weaknesses and resistances, usually giving extra combat turns if you exploit a weakness, taking them away if you screw up and use something the enemy is strong against, and often taking that even further. Combat changes a bit by game, but it's generally pretty strong and fun, honestly. And also tends toward the "oh gods what are you trying to DO to me?" levels of difficulty, for at least some fights in most of the games.
The games also tend to be pretty solidly JRPGs, although different games may have other genre influences as well. The protagonists are also usually young - although "young" in this case can be anything from younger school age to young adulthood.
Anyhow, I guess that's enough about the series as a whole if I'm going to talk about various specific games, so let's get started on that. Keep in mind that the games I haven't played at all, I won't talk about, so there will of course be some missing from the list.
The Persona sub-series:
The Persona games are a sort of sub-set of the SMT games which have some particulars in common. For one thing, all of them have high school students as the protagonists (or at least some of the protagonists), for another, the demons are grouped up by Tarot categories (Empress, Chariot, etc.) Um, although come to think of it, I can't remember if other games were like that as well, or not. *cough*
Anyhow, they also use demons as "Personas" of the characters, alter egos or reflections of some aspect of their personality. These Personas are summoned briefly in combat, rather than wandering around on their own, in order to cast a spell, make a special attack, or what have you. The equipped Persona also effects the weaknesses/resistances and the stats of its paired character, making them important for more in a fight than just a quick spell.
They're also probably my favorite of the SMT games generally, and if I had to pick between an unknown new SMT general game, and unknown new Persona game, knowing nothing more specific about either, I'd go with the Persona game. (And then do what I had to to get the SMT game as well later on. :p)
Persona 1 and 2:
These came out originally on the Playstation and were brought to America, though one has also been re-released/remade slightly on the PSP. As for two... er, that's a bit... well. The game was broken up into two parts for whatever reason, "Innocent Sin" and "Eternal Punishment," and for whatever reason, only Eternal Punishment was brought out originally in the US. And Innocent Sin is slated for a PSP release in September over here, finally... but not Eternal Punishment. Although they could always still announce a release for that. So anyhow, it's all a bit... weird.
Anyhow, I don't tend to like these games as much as 3 and 4 of the series, not only because they're dated (which I can live with), but because I like what they've done with the gameplay of the newer games more. The first two use more typical old-school JRPG combat systems, with all the characters able to use any Persona (I believe, or at least they each can link up to multiple Personas), and the games themselves take place in the real world, often in the schools, where "weird stuff" is happening.
The stories are interesting, and IIRC the games aren't overly long, but I could be wrong. It's been a long time since I've played either to completion, although I've played part of the first game on the PSP. As I said, I prefer the other two.
Persona 3 and 4:
These are both more recent, both being put out for the PS2 later in its cycle, and they have a rehaul of all the systems in general. And, as I've said, I think they're both more fun.
For one thing, while "weird stuff" is going on, and the town knows there's something odd, the real magical/supernatural stuff is limited to other... places, perhaps you could call them other plains, that the characters can access and explore. So you can go between normal life for the students on the one hand, and then the supernatural stuff on the other, without them overlapping too much.
And that means... it's schooltime! No, seriously, each game is structured around nine months or so out of the school year, where your main character wakes up each morning and does, well, normal high school stuff. Going to class, going to clubs, shopping... whatever. And, in between, doing exploring of Tartarus/the TV world, depending on the game, and managing the various Personas. You know, typical student stuff.
And this is actually important, because a) it means there's a limited amount of time to do whatever it is you want to do, and b) there are separate stats you can raise such as courage, knowledge, etc., that have effects on what else you can do in turn, which you can only raise at certain times through certain activities, and c) there are also Social Links, which are based on the strength of your friendship with certain special individuals, and these Social Links strongly affect the Personas that you create through merging other Personas. So, yeah, while normally I'd be the LAST person who'd want to go back to school, in this case it's actually a lot of fun!
The fighting's also different, even aside from taking place in a different "world." For starters, only the main character can actually summon different Personas, with the other party members each having only one specific Persona (although that may evolve through gameplay to become a stronger one), getting stronger just by leveling up. Also, from what I remember the combat makes more use of weaknesses and resistances, and does some different things with that (like, if you knock all enemies down with criticals/weakness exploits while you have your main character and at least one other party member still standing, you get a free gang attack on all of them, usually for a lot of damage.)
Frankly there are more differences, and more things to talk about with each game, but since this is supposed to be a semi-brief overview of SMT games in general I'll stop there I guess. Just reiterating that these games are fun, and quirky, and if you can stand JRPGs at all (or don't know if you can), you should try them out.
Shin Megami Tensei: Nocturne:
This was the first SMT game I played outside the Persona series, and is still one of my favorites, although I don't know how well it'd hold up today, it being a while since I've played. It's another PS2 game, but one coming out well before Persona 3, and thus a bit more dated. Still, I suspect that the gameplay would hold up well.
In this game, you don't only consort with demons, you become a demon (or half-demon, "the demi-fiend") yourself. And rather than partying with other students, your party members are the other demons you meet and form pacts with, themselves. Since you're not summoning and bonding with demons, your strengths and weaknesses in combat come from the party member lineup you choose, and from the "megatama" that you choose to fuse with, which determines your elemental affinities, what spells/skills you get, and so on. These in turn are found, purchased, given, won... however you can get your hands on them.
There are still some other characters (human and otherwise) that show up throughout the game, they're just not your party members. See, the storyline behind this game is that at the start, the world becomes changed or destroyed depending on how you look at it, and the main part of the game is a sort of waiting period between the normal world and the next, while it's still up in the air as to what sort of new world will be created. And your main character, of course, ends up influencing this. Depending on your choices and your actions/behavior, you end up on the path to one of a multitude of new worlds, each different from the next, and each with a "proponent" of sorts that you know and deal with outside of combat situations.
The game itself is generally straightforward JRPG from there, with of course the demon pact/merging/etc stuff added in, although there are a few interesting sidequests/minigames/other things you can do. Still, it's an interesting game, and recommended despite the age.
Since I've started with the PS2 games, might as well keep up with them...
The Shin Megami Tensei: Devil Summoner games:
These are two games (well, okay there were apparently others for the Sega Saturn, but I have no idea about those), and I've only played partway through the first. (Er, and looking at the data, the second actually isn't out yet over here! Oops!) I didn't hate it, but it just didn't grab me like some of the other games, for whatever reason.
These take place a bit earlier in the history of Japan, with your main character being the next in a long line of Devil Summoners, who can see, fight, and form pacts with demons. Your character works with a parter, a private investigator of sorts (with a bit of a noir/Japanese feel), at least in the non-combat situations. It's your job to help investigate various supernatural mysteries, and put things to rights.
The gameplay for these (or at least the one I played) is a bit different from the others. For starters, your demons are more useful outside of combat - you can summon one at a time to follow you around, although normal humans can't see them. Each one of your demons has a certain skill (or possibly multiple skills) that you can make use of out in the streets - for instance, despite the fact that humans can't see them, some may still be able to manipulate the emotions of the NPCs you talk to, revealing new information. Things like that.
The combat is also different - instead of being turn-based, this is an action-RPG, with you and a summoned demon fighting together in real-time, using a sword, gun (with special elemental bullets), and spells/abilities. I think honestly this might be one of the things I didn't like as much about this game - I tend to like the combat of the others better, not to mention the larger parties and larger supporting cast.
The Shin Megami Tensei: Digital Devil Saga games:
Like the previous sub-series, I only played part of the first of these. Now, I didn't get into it as much, but I believe for a lot of people they're some of the best of the larger series, so to each his own.
Anyhow, this takes place in an alternate world or future world (I forget which, I think the latter) with warring gangs vying for control of territory. Your main character is the leader of one of these games, with other party members being your underlings. In this, you gain demons that are connected to you, somewhat like Personas, but here you can turn into these demons rather than summoning them in combat, and you gain new powers and experience by cannibalizing the demons you defeat in battle. Yeah.
The combat is a lot like in the latter Persona games (and the Nocturne game as well), but the main difference IIRC was in how your demons developed. I don't think they changed, or at least their transformations were more limited, and your characters' strength was more based on leveling and choosing how to develop powers, than on the new demons you could form pacts with and create as in the other games.
Really, this is a bit of a departure from the norm for the series, and I should really try to give it another go at some point. Sometime.
Really most of the SMT games I've played have been for the PS2, but there are a couple others left that I've tried:
Shin Megami Tensei: Devil Survivor:
This game is actually for the Nintendo DS, and a bit different in various ways. But fun!
The story here is that you have seven days to find your way out of... a tricky situation (to try to balance between description and spoilers), with a twist: you have a special program that lets you see some items of news and interest the day before they happen... or were to happen. It lets you see major events that could happen, and then try to twist them or prevent them, to get a better outcome.
Your characters this time are high school students again, although the game itself doesn't revolve around school in any way, like the Persona games. At least, your first party members are - you pick up other members as you go along, depending on your actions and choices.
One of the big differences in this game is the combat system. It's a tactical RPG rather than a straightforward turn-based or action RPG, with each human character leading a "squad" of themselves and two demons who in turn help and protect him or her. You can choose up to four of these squads to have on the battlefield at once, while the enemy will also contain squads of demons moving around the battlefield. Squads can heal each other (with the right abilities/items) or perform other actions while moving around. When opposing squads meet up, one may attack the other, which leads to a round of attacks, or possibly more if use is made of elemental weaknesses. When all of one side's squads are taken out, the battle is over.
There is also the fact that you don't form pacts with demons quite as in the other games - instead, you "buy" them at special online auctions, although you can them merge them as well. Your human characters can also get and use abilities through use of "skill cracking" in battles, learning the skills from enemies that have them (if I'm remembering and refreshing my memory through Wikipedia correctly) and then assigning them for their own use.
One of the things that makes this game stand out is the fact that there are multiple different paths you can take, depending on where you go and what you do throughout the seven days. See, there are often different things happening at the same time in different locations, and doing many of these actions advances the clock so that you have to choose between different activities. And what you do at one point helps to determine what you know, and what you can do later on, including saving and recruiting different party members. There are different distinct endings to the game, depending on what you've done and what exactly you decide on as your end goal, and figuring out how to get these and have things fall out in the best way possible is challenging and fun.
Shin Megami Tensei: Strange Journey:
This is the second SMT game for the Nintendo DS, and... isn't as fun as Devil Survivor. Really, this is another one that I played part of then stopped, because I just couldn't get all that into it. It's kind of a dungeon crawler, from a first-person perspective, and... I think that's one of the reasons I didn't like it much. You can wander around base a bit and buy some things, talk to some NPCs, but generally you just are at base, or are wandering around exploring. It's... a bit dull, for an SMT game! And also a bit hand-holdy, if I remember. But honestly, I don't remember all that much!
And frankly, I don't feel like trying to wrack my brain too much about it because this is already more than long enough. It seems like there should be more games I've played, but... maybe not?
So anyhow, pretty much any of the games I would say are interesting and worth trying, just perhaps I've been spoiled by the better ones in the series, and haven't given the others a fair shake. :p
If there's any specifics anyone wants of some of the games, or what have you, leave a comment!