I have one. I works well for what it is. Don't buy the Veho-branded one though, you can get the same device under different names from places like dealextreme.com for $20-30. They're all Chinese-made.

The good: surprisingly sharp focus and reasonably fast exposure auto-adjustment as lighting conditions change. Mic has decent sensitivity for recording audio (though it's usually swamped by wind noise when used on a bike).

The bad: Dynamic range for contrast is limited. No image stabilization. Only a blinking LED to indicate whether it's recording or not; i.e., no audio feedback when used as a helmet cam. No immediate way to tell if your aim is correct, you can only see after the fact when you download the recording to your computer. No plug to cover the USB port or the microSD slot to protect them from weather. Battery only holds enough charge to run 1-2 hours. Standard package only has spring-clip mount.

Untested: VOX (sound-activated recording) -- it's pretty pointless on a bike as you have constant wind-noise while you're in motion.