For mountain biking any mount attached to the bike is gonna be kinda shitty, there is just too much shock. I've gotten some great helmet cam footage with my Contour even on a hard-tail bike, part of the key was making sure the helmet was very tight around my head (and therefor did not rattle). Noticed a big difference between footage shot with a semi-loose and tight helmet.
I've got a road bike and mountain bike edit I'll share once I get them uploaded to youtube.
I wasn't disappointed with that footage (regular 720p I think) when using the Contour flex strapped to the down tube or the top tube, those are relatively stable. GoPro mounted to the bars also returns decent footage sometimes. The helmet cam was too damn wobbly, though, I suppose I should tighten the strap but it just doesn't fit snugly like my snow helmet.