I bought a 808 HD 720p from ebay a few months ago for about 40 bucks shipped.
It works well for what you pay for. I can understand the appeal when using them on RC cars and rockets etc.
But for mtbing you can do so much better.
Its not a bad camera, just not that great. Its supposedly one of, if not the best of the bunch.
In Bright day light it does a good job. If your flying down a track which is covered with trees and there is shadows everywhere it lags.
In low light it really freaks out. Will lag and even skip frames.
The main issue i had with it was the Focal length. Or lack of. The view is far to narrow. (as you can see in my videos below)
As its a pinhole type camera, it has trouble with the edges of the picture getting dark.
Below is a Bright Sunny day with the 808. (Both cameras mounted on my helmet and in 720p on similar days)
http://www.youtube.com/watch?v=0vIQBOmmBgM&list=UUJgaDWCmftpfatMmRUm4CNQ&index=3&feature=plcp
Here is the same couple of jumps with my GoPro2. (yes i know the GoPro2 is completely different yada yada)
http://www.youtube.com/watch?list=U...feature=player_detailpage&v=Be-x337Bst0#t=31s
The worst was a DH run with shadows and trees all over the track. The 808 flipped out. It wasnt fast enough to change between sun and shadow.
Then as a result is lagged and the reso dropped to like 200x200 (from 720p) a few times.
One last thing to point out is the mounted options for the camera...they stuck....you have to DIY it.
As someone mentioned before
http://www.techmoan.com/ does great reviews of all the cameras out there and which buyers get them from.
Hope this helps