Theatrhythm was definitely a game that I thought was too cute for me. I honestly didn't think that I'd enjoy it as much as I did. In fact from release for about 45 days I've played it for an hour daily during my commute (I skipped a day, lost my bonus and didn't pick it up [or the 3DS for that matter] since).
The game has several hours worth of gameplay and since I'd preordered the game, it came with a stylus and some stickers. The stylus was actually quite handy for the game as it had the right amount of touch sensitivity and length that's required for a rhythm action game.
The game comes with 36 tracks unlocked that can be played in blocks of three in the 'story mode' or as individual tracks. There's also about more than 10 tracks that are to be unlocked and if you're willing to pay 90p per track there's more to be bought via the ingame store. The game initially gives you 12 characters but collecting gems through chaos notes or through levelling in the game you can unlock a further 17 characters.
There's definitely a lot of replay value in this game. It's all about trying to get near perfect scores and beating the chaos notes, the latter of which can get very hard as you progress. Every 500 points you unlock something - a crystal (For a character unlock which requires 8), things for your game specific streetpass id and at specific scores you unlock new music as well.
It is quite clear that they've given more music and other features for the FF games that are popular such as 7,8 and 10 but that's alright and understandable. They do try to put a fair number of stuff from 13 which didn't have bad music for that matter. If there's a gripe it would be that I wish they put higher quality music for the earlier FF games which do sound a bit midi-ish.
All in all if you're a fan of rhythm games or of the FF series I'd recommend the game as it's a lot of fun. I enjoyed it at the preorder price of £22 and I'm certain it's more than worth it at a lower price.