Hey, Only played it for an hour or so, but i'm really enjoying it. I don't speak/read Japanese, so i'm sure i'm missing out on some hillarious antics during the scenes in the club room with the K-On girls, but outside of that, the game is pretty much easy to navigate/play.
I've unlocked ten of the songs so far (played about 6) with there being a total of 19 songs. Each song allows you to play as one of the five girls/instruments, and each one is different to the other in terms of button pressings, timings etc, as you'd expect. Ritsu is my favourite so far (drums) but that's just because i love the drums. Apparently she's the hardest to master!
Once a song is beaten on normal, you unlock a hard mode, and after that, I believe there is a hard+ mode. So really there are 19 songs, 5 variations of each song (90 plays) and 3 difficulties (270 plays).
If you're a fan of K-On i'd easily recommend this. Great songs, fun gameplay and loads of additional stuff to unlock (clothing - Dress up the K-On girls!, decorative items for the various locations (club room, music studio at Mugis beach house etc, food items that affect gameplay)