I've used a number of (mainly Contour) bar mounted cameras and you can get good results. The main problem is vibration, not only off-putting to watch but it also can cause rolling shutter wobbly (jello) video with cmos sensors. This can be minimised using very wide angle settings, which sets foreground oscillation movement further into the background, minimising its amplitude and rolling shutter effect. This and using 60fps (higher sensor scan speed) can give some pretty good video - generally this means using 720p video, 170 degree fov, 60fps.
Watch out for fov claims - a cam may be advertised as 170 degree fov but only achieve that at 720p, with 1080p at 130 degrees only. Some cams are now appearing with 170 degree 1080p video at 60fps. That must fill up a memory card pretty quickly.
The other problem is that bar mounts have to be absolutely rigid to avoid road vibration being exacerbated by mount vibration. I've always ended up making my own as supplied ones have adjustable joints that, no matter how rigid they appear, just make road vibration worse.
GoPro are the market leaders but the ContourRoam2 cams are pretty good and less nerdy-looking. Here's some video (40mb) from an older Contour cam that has been re-rendered in Dashware:http://whimsy.myzen.co.uk/example/melton.mp4