To give a little more detail, the game essentially consists of a) story, which can pretty much be skipped, and b) battle, which is done via playing the melody parts of various reasonably catchy songs. The person playing the melody at the moment (competitors take turns) is considered to be "attacking". A blue path approaches the center of his screen from varying directions; he has to a) press down the "play" key when the start of the path hits center, b) follow the path as it curves, and c) let up on the play key when the end of the path hits center. The degree of accuracy he maintains in doing this determines how strong of an attack he gets, and sufficiently poor playing will hurt him (in multiplayer, anyway; in singleplayer, attacking is equivalent to healing). The person not playing melody has the job of dodging the incoming music (picture the music as Edward of FinalFantasyFour attacking), which he does by pressing various buttons as their visual analogues hit the center of his screen. Obviously, if he fails then he takes damage. If both parties succeed, then the tie goes to whoever succeeded better (i.e. better timing). This is one of the games where you really don't want to pay attention to what's actually going on in the background (i.e. renderings of your character playing loud flashy music and the other player jumping and dodging all over the place), as you really need to pay attention to the incoming paths/symbols.
It's a lot of fun, and the music is pretty good. The only major problem I have with it is that it can be very difficult to finish multiplayer games sometimes, as a mild mismatch in skill typically results in one or the other team dropping out well before the song is finished. Certain songs (e.g. the last one in the game) have yet to be completed in multiplayer, although they're not horribly bad in singleplayer (since there's a more effective way to heal yourself). Okay, so the last song is pretty tricky, but it can be done at least.