Why a helmet camera? I think something mounted to the bike would be much more stable, and therefore easier to watch video recorded with it. I have a handlebar mounted unit and can't imagine switching to a helmet cam.

That said, I have seen some awesome race footage recorded with helmet cams, the POV really makes you feel like you're riding the bike. But other than for that, I think handlebar-mounted or rear-facing cameras are much more practical for everyday use. The wattage output and miles per hour display this guy has set up is really awesome, as is his brilliant win.