Holy crap, Beats
[link] rocks my socks.
I mean, wow. First off, it's only $5 and a 200MB download from the PSN. Second, it auto-detects all music on my memory stick and generates a level from ANY song with four different levels of difficulty. I could rock out from ABBA to Dethklok and it would just work, it's amazing and way fun. I'm loading my PSP up with songs right now to get more out of it. Faster songs are, of course, more difficult than slower ones and usually more fun. "The Pretender" by the Foo Fighters was especially tricky at first, it's got a very quick chorus.
There is plenty of customizable bits, like theme, wallpaper, and visualization. It keeps track of your top scores for each separate difficulty level and the percentage of perfects you get per each song. There's community modes via infrastructure and ad-hoc multiplayer that supports up to four people (but I haven't been able to make use of this yet).
My god. If you have a PSP and like music/rhythm games, go get a PSN account and download this game.