I picked this up yesterday. Haven't played it much but for the few songs I did play it for I had a huge grin on my face. [You must be registered and logged in to see this link.]
It takes a song of your choice (it can do this for a number of formats) and turns it into a track not unlike the ones you see in Wipeout HD's Zone mode. Slow parts of songs are uphill while fast parts are downhill.
If you don't happen to have any songs with you to play, it comes with Valve's Orange Box Soundtrack.
If you don't want to shell out the $2.50 right away even though I highly recommend you do, there is a demo available for download.