Wow, I was just playing this game and thinking about posting about how awesome it is. Definitely worth $6. If you're into rhythm games at all, this game will totally satisfy you. The fact that you can choose to play whatever song from your memory stick that you want is very cool.
I only wish you could create a playlist and just play a bunch of songs all the way through instead of having to return to the main menu after every song. Other than that, it's just great casual fun.
P.S. I'm loving the PSP right now. I'm not buying a PS3 game for a while because of all the good releases on the PSP coming up: Wipeout Pulse, Patapon, God of War, Crisis Core all out in the next 2 months.
My Top 5:
Shadow of the Colossus, Metal Gear Solid 3, Shenmue, Skies of Arcadia, Chrono Trigger
My 2 nex-gen systems: PS3 
and Wii 
Prediction Aug '08: We see the PSP2 released fall '09. Graphically, it's basically the same as the current system. UMD drive ditched and replaced by 4-8gb on board flash memory. Other upgrades: 2nd analog nub, touchscreen, blutooth, motion sensor. Design: Flip-style or slider. Size: Think Iphone. Cost: $199. Will be profitable on day 1.
