I just played the demo and it is soooo awesome. I'm seriously considering buying it but since I've already spent £60 this month and I wanted to save nearly twice that I'm not sure I can afford it.

The demo's pretty clever, it's basically the whole game but with only 4 run-throughs allowed. I managed to hit the top 10 on the first song I did (Pulse of the Maggots by Slipknot) which I'm pretty proud of.
The whole game just works so well. 'Mellow' songs are represented as uphill sections with 'cool' colours such as blues and greens, while more, uh, exciting sections are represented as being downhill with red and oranges. On something like Pulse of the Maggots there are a couple of slightly less intense bits which still feel fast, so you're thinking 'ah this is pretty tricky' then suddenly you're in the chorus blasting downhill. It's seriously the most grintastic game I've played in a long time.