Really liked it! A bit on the unforgiving side, but not too bad IMHO.
Jumping felt a little bit strange at first, but when I got used to it I think it worked quite okay (air-jumps aside of course).
Tried at work (so I couldn't play it as long as I wanted to ), and it run okay here where most things runs choppy. (WinXP, Core2Duo)
All-in-all a simple but very fun game - two thumbs up. Wish I could keep mine as simple and yet so entertaining