Holy macaroni! What if suddenly, your Windows Media Player or iTunes visualizer came to life?!? All those strange spirally lines and colors, strobes, all flashing and inducing epilepsy and pointlessness suddenly became a video game?!?
Which brings me to a strong concern. The game works well when played in short 3-4 minute sessions, on a whim, in between other more important gaming titles, or at break from work. And when you want to LOOK, not just listen to your music. But beyond that, the game has little sustainable value. Sure you can tell your friends that you rode the Dragonforce wave and scored some serious points on Audiosurf, but then some other dude with a console can easily say he did the same on Guitar Hero Expert, and the crowd will go nuts for that. This game's not big enough for people to care about, whether you score highly or not. So what's the game left with, other than you entertaining and playing by your lonesome self.
Also, Audiosurf's got a sweet deal with Valve in that they've got the entire Orange Box soundtrack (i.e. Portal's genius "Still Alive") thrown in, so you can hear GLaDoS go "this is a triumph" whilst you put pedal to metaphorical metal. But only if you are a fan of Valve's games anyway. If not, you can always play Britney Spear's Toxic, and it'd be just as fun... I think - I have not tried with that song personally.