Daigasso! Band Brothers DX is totally rocking me right now. I got it (imported) a few days ago. It's much easier for me to get giddy over the songs in here than in something like Guitar Hero, cuz I'm less of a rock-head and much more of a Japanophile. The game kind of requires you to be able to get online with your DS, since that's where all the (good) music comes from. It comes with just over 30 songs that are mostly 'classical' pieces, but the game launched with 200 downloadable songs from Nintendo. Since then, people have been uploading upwards of 30 user-created versions of real songs every day. Here's some stuff I've downloaded:
Hare Hare Yukai - Haruhi Suzumiya
Bouken desho desho - Also Suzumiya
Sora Iro Days - GURREN LAGANN
Ride On Shooting Star - FLCL