Taiko no Tatsujin DS is fun, and has a nice selection of Japanese tracks, but it's awkward to play. IMO, at least. It's a neat little game that has tons of replay value if you're into it, and the menus are very easy to navigate since the game doesn't require much (if any) knowledge of Japanese.
I'd also like to throw a nod out there to Electroplankton, developed by Indies Zero and published by Nintendo. It's not so much a music game, per se, but it's a piece of software with no real objective other than to create music through arbitrary and random means. The only downside is, you can't save or record any of the music you create. Unless, of course, you do a line-in recording or video tapes it, or, whatever. It's one of, if not the most innovative title out there on the DS, and makes complete use out of the system; really something that couldn't be replicated on any other console or handheld.
Trailer:
Jam it back in, in the dark.
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