I see a ton of people with those at the track. They're easy and produce decent video, but the sound is muffled and crappy, has no zoom and is bulky. But it's rugged and cheap enough that if you crash you won't cry.
vid with a Gopro HD:
http://vimeo.com/6562596
I run an Aiptek camcorder with a Helmetcamera lipstick cam velcro'd to my helmet. It works well and is compact, but the mic picks up a ton of wind noise and the input jack on the camcorders tend to go out.
What my rig looks like (hard to see in the pic: camcorder is in the fanny pack and the can is on the rear faring pointing back)
Vid quality:
http://vimeo.com/5152798