Started playing Theathrythm recently (after having to go through some trouble to find a copy, it's sold out at a lot of places) and I'm absolutely loving it!
I'm not much of a rhythm game fan, since I tend to suck at them, but Theathrythm is absolute bliss. It helps, of course, that I know all the music and that it's just plain damn fun to tap along with the music I've known and loved for so many years (seriously, why are there not more music games with actual game music!?).
At first, only Basic Level difficulty is available, which no one really should have trouble with, but afterwards, Normal Difficulty becomes available, which is a lot more hectic, and has just the right amount of challenge.
And then there's the 3rd difficulty which is absolutely CRAZY, but feels OH SO GOOD if you manage to get through a song unscathed.
The RPG element is surprisingly fun too, with stages not only having a length determined by the duration of the song, but also having a "physical" length, in which progress is determined by how strong your party is. A weak party will not get very far by the end of the song, but a strong party might end up at a boss which, upon defeat, may drop cool items and collectibles. It's a very fun and interesting system, greatly increasing the replay value of songs.
The streetpass function is used to exchange profiles, but each profile comes with a musical "score", which is basically a randomized piece of two songs from the game at random difficulties, with random monsters and bosses thrown in which, of course, will randomly drop loot. The game touts this system as offering "virtually endless possibilities" and it's definitely something to bring you back after you've worked through the Story Mode.
Did I mention that?
Yes, there's a Story Mode. Basically each FF gets 5 songs taken from that game: The opening, an event scene, a battle scene, a field scene and the ending. The opening and ending aren't that interesting from a gameplay point of view, since they're only for bonus purposes and there is no fail state, so it's fortunate that they can be skipped (a wise choice, since the opening is the same song for many FFs: the Prelude, obviously).
The battle and field scenes are the main meat of the game, and they're where you can earn the previously mentioned items and loot. Progress in battles depends on the strength of your characters, while field scenes depend on speed and a bit of luck. Your stats obviously don't influence your performance in the song, but it's nice to see your performance in the songs be translated into collectible items.
As far as the music collection goes, you'll get most of what you expect and a couple of less expected ones. Each game uses the music from that actual game (so no remixes), but the various menus all have nice arrangements of familiar FF tunes.
This may very well be the best music game I've every played in my entire life.