Gopro Helmet Hero Wide is very good- adequate at everything and there's a good range of mounts. Also runs off AAAs rather than needing its own specific batteries, which is great. Have to say that the mounting isn't as solid as it should be, the shape of the camera means that on a helmet mount it's inclined to rock back and forth even if the straps are seriously tight, so a wee bit of tape can help that.
The biggest problem with mtb videos is that on a helmet mount it'll generally get rattled about since your helmet will tend to move.
The thing to do just now, if you can, is wait for the Hero HD, it's probably not far from market and it's going to be great, though it does need its own specific battery which is a pain.
If it says "Oregon" on it, ie ATC, it is s**t.