I rented a GoPro a few weeks ago and was very disappointed with how they perform on a sunny day with light coming through the trees. Bright bits are horribly overexposed to the point of making it painful to watch! Also whilst the chest mount gives the best perspective, it’s so rattly and shaky in that position if you’re going over any kind of rough ground. I think helmet mounted and on an overcast day it would work better. They also flatten everything, make you look slow and make jumps and drops look non-existant. Thing is from what I’ve seen the GoPro is the best one about in terms of sharpness. I wouldn’t waste money on one for mountain biking, personally!
youtube channel here (videos there have been heavily compressed, the stuff straight off the camera is much sharper).