My spacebar is on the fritz, bear with me...
Anyways, anyone else play these? For the most part they're all for the Playstation2 (at least the ones that I own/have played), but there are several older games (and 2 newer ones) that you can play with an emulator and roms/isos (is it forbidden to talk about those here? never saw it in the rules).
The whole idea of the games is on demons and the supernatural, and most of the games have deep and dark storylines. There are several offshoots of the main series, including Persona, Devil Summoner, Demikids/Devil Children, and Digital Devil Saga.
Anyways, I'll give a short rundown of the games I know about:

Shin Megami Tensei I: I started playing it, and I'm stuck. It seems pretty good for an older game, however. XD

Shin Megami Tensei II: Never played it, however, I take it it's much like the original, as it's for the same game system (SNES). You can get Roms for these two.

Shin Megami Tensei Nocturne: I'm getting this game in the mail in a few days, hopefully. From what I've read about, it sounds amazing. It tends to run pretty expensive though (I got mine for $80...). For the PS2.

Devil Summoner: Raidou Kuzunoha vs The Soulless Army: My first 'true' Megaten game (more on that in a bit). The battle system in this is a fresh break from other Megaten games, as it's not turn-based. It's a whole lot of fun, and there are a ton of demons to get. It's still at a reasonable price on Ebay and game stores, as it's a newer game.

Demikids/Devil Children (Dark and Light): The gateway drug into Megaten, for kids. XD It's not that bad, actually. This was my first Megaten game. It sticks to the roots of the first 2 games, and is a bit like Pokemon. You can get a Rom for this one (it's for the GBA)

Digital Devil Saga: Much fun for RPG fans. Great storyline, too. You turn into demons and fight stuff. XD Trust me, it's better than it sounds. There's also a sequel. For the PS2.

Revelations: Persona: The first Persona game. I'm downloading it right now, as an ISO for the PSX. Don't know much about it. Will explain more when I get to P3.

Persona 3: THE game that made me a Megaten addict. Schoolkids summon spirits by shooting themselves in the head. Yeah. The characters are all amazing, the battle system is fun, the story is great... there's really not much else to say. It's probably still out in stores, so get it while it's hot!

Yeah... that's my addiction, and I'm glad I could share it with you. Megami Tensei draws heavilyl on different kinds of mythology, so I reccomend it for the mythology fan. You'll learn a lot, too.

Oi! I didn't think anyone else was into this series! I'm not alone! Yays!

Unfortunately, the only ones I actually own are Persona 2: Eternal Punishment, Persona 3, Nocturne, & Digital Devil Saga.

I've emulated all the others, but my favorites are P3, P2EP (Tatsuya's awesome XD), & Nocturne (Dante FTW).