Bike mounted cameras vibrate like mad. Sometimes so badly that you can barely tell where the cars are, though this depends on the conditions of the street and the type of bike/tire you run.

I have a Contour Roam3 on the helmet (VERY much more smooth and easy to view the video) - I really dislike the GoPro brick format, it's horrible for mounting on a helmet and not very pretty in any situation. On the back I run a Fly6, which is IMO an absolutely ideal rear camera. The only caveat on the Fly6 is that the batteries seem to last about 2 years then they're shot, but I was able to easily replace the battery on my for about $5 and an hour's time.