Rather than a port of the console version, this game is really a follow up to Frequency, a PS2 game, and it works brilliantly on the PSP.
Essentially you control the whole band. If you complete a phase (a series of notes) than that instrument keeps playing, and you then use the shoulder buttons to move to a different instrument. The aim is get the whole band and song playing, with more points for hitting successive phrases.
The whole thing works really well on the PSP - I've been playing it continually. The difficultly level is also really well judged, so the game is never frustrating. The tracks are also well put together, so, some what strangely, it really does feel like you are playing.
Definitely recommended - it's my favourite PSP game.