A helmet cam is a good idea. I rode a year without and a year with. The difference in how cagers respond once you point to the cam is hysterical. I actually had decent footage of one driver looking in his side mirror. He clearly saw me in the left lane and was grinning as he started over into my position (rush hour so he knew he was squeezing me into the jersey barrier), then saw the cam and sank down in his seat as he backed off and slowed to get away from me. All on the footage so no way to tell the judge "But Your Honour, I just didn't see them".
I have a Contour and have read many good reports. My experience has not been good so I've just gotten a Drift HD and will put it on the helmet I'm getting. I hear good things about the GoPro but haven't tried it myself.
My purpose for a cam was for proof in case something happened rather than to document my rides. I wanted a helmet mount so I could pick more around me than a fixed mount would do. For that reason I didn't try the GoPro since it seemed better suited to a bar, tank or chest mount.
The Contour mounted well to the side of the helmet but follow the instruction to clean the surface and let the adhesive set. The bezel will rotate and the tilt can be adjusted so pay attention to the pan (side-to-side) and you can get a good field.
I looked in a mirror and slide the base along to where the cam pointed forward/center then marked with a grease pencil.
I added a "safety leash" shown below and that served me well since the cam slide off the rail once and the cam disk separated from the base disk once.
I made a small loop and put the strap though it, then put the cheek pad back in.
The footage is generally clear but licence plates are not as easy to read in the film as one would think. I know the TV police dramas are mostly bull but I hope they're right about refining an image to get the numbers if needed. For now if I feel like I might be wanting those numbers I try to get a shot at a stop light so I can get a good, close, less vibrated image.
Hope this helps.