I use Contour HD and Go-Pro HD. The example.
Monster tail chop videoI like Contour HD better, mostly because it's easier to operate. Push the power button on, slide the rec switch to rec position. That's all, couldn't be simpler. If "riding" is a main thing (and you just want to capture some footage for fun / sharing with friends later ), you don't want to deal with too many functions / modes . (I have done it, others have done it. WIth Go-Pro, it happens that you thought you were shooting video and actually taking bunch of photos ...) That said, Go-Pro isn't that bad a choice.
Just be careful, sometimes mounting a camera on the bike has the unintended side effect of lowering your IQ
+1
Some camera's make everything look too slow as fps is to low I guess.
It also depends on lens (how wide), camera position (how close to the ground, how it's angled), and editing. Basically, everything you would see on video (if they are shot on the street at sane riding pace) WILL look somewhat slower than you felt while actually riding the bike. You can alter the speed (to make them look faster ) at editing stage (although I never bothered to do it.) See first Mad Max movie, it's a great example of how to shoot and edit with low budget.