I started off with an ATC5K a few ago which worked well but the resloution wasnt brilliant and the FOV was pretty limited:-
ATC5K Helmet Action Camera by Oregon Scientific
I then moved to a VIO POV 1.5 with an 8gb SD card which I have mounted to my headset:-
VIO POV 1.5 Camera System - PointofViewCameras.co.uk
For me the main advantage of these cams is that they have an LCD screen so you can see exactly where they are pointing, its very frustrating to do a flight then come back and review the footage to find the camera wasnt quite set up right and its at a funny angle or pointing mostly at the instrument panel.
There are now better options on the market and POV have a new HD cam which would be my choice if I was going to change:-
POV HD Digital Helmet Cameras :: V.I.O.
A pal of mine uses the latest GO-PRO strapped to his head when flying at work and that also produces some excellent footage however unfortunately I dont have enough room in the cockpit for something that bulky and am limited to bullet cams...
Regards
UA