When I was training towards my PPL my instructor used the NFlightCam - initially I was really impressed with the quality. The only problem he found was that after about 3 months the camera started to corrupt files willy nilly! The camera was being used quite heavily I should point out though, so maybe this is a unfair review.
I use a GoPro Hero 960p - the slightly cheaper version of the full HD one. The quality is great and I always wear it attached to the head strap. The only problem I've found with the Hero is that it struggles to adjust from constant variations of light. If it has focussed out the window it takes a good 4-5 seconds to adjust to a cockpit when focused upon. Saying that I love it to bits!