I"d like to toss out +1's to both Diagasso! Band Brothers and Rhythm Tengoku Gold.
Both are stunning, stunning works of rhythm games. D!BB is, as stated above, a great, albeit lite (but its NOT) looking rhythm game. It looks simple, even sounds simple, but has the greatest depth of any rhythm game out there. Also, you can create your own tracks on it once you beat it. Well, you can from the start, with an extremely gimped editor, but when you get far enough in, you'll unlock a full staff editing section. Truly a marvelous rhythm game.
Rhythm Tengoku Gold. Wow, this is an odd, excruciatingly fun game. Take a rhythm game, give it the love of Wario Ware, and toss in some funky beats, and you have RTG. Its short mini-games done to beats. Screen taps, quick motions with the stylus, etc... all done to on-screen cues (never the same in each level) and to a beat. Then, after a few levels, they'll mash up the last 5 levels into one song, and change back and forth between them... all to one ongoing beat to make a full song. Bizarre and addicting, a VERY original rhythm game that will have you sucked in for hours... w/out realizing it.
Diagasso! Band Brothers Deluxe has also come out (JP only), and its a lot of fun as well, but it doesn't have the simple charm of the first one... so check out the first, then snag Deluxe if you like it!
ps D!BB (the first one) was one i actually imported before i found the wonders of flash carts, and holy hell if i don't regret buying that one for a second. Now i have no idea where the cart is, so i'm thankful for the flash carts heh. But yea, D!BB is great.