the only light i'd bother putting on a helmet is a red-zone - http://www.niteflux.com/store/

all the time i see people with lights on their helmets... pointed at the sky, pointed at the ground, hidden by their backpacks, etc. putting a light on your helmet doesn't "make people see you"!

http://www.youtube.com/watch?v=tckl9f3T5xc

NOTHING comes close to the red-zone for off-axis conspicuity. take any of the other lights mentioned above, in this thread, and off-axis conspicuity drops to near zero. off-axis "visibility" may be there, but those lights are merely "visible", not conspicuous, off-axis. outside of their narrow beams, they don't demand attention.

so unless you can keep your head pointed so the small "hot spot" of the light's beam is always pointed the right way, save up for a red-zone.