I just got my UltraFire C3 Q5 last night, and rode in with it today. I went with a single 14500 cell for today's test. The light output was sufficient (not wonderful, but sufficient). The big problem was that it cut out at about 30 minutes. I could turn it back on again but it'd shut off again quickly. Then I put it on medium output instead of high, and it lasted the remaining 10 minutes of my ride.
Tomorrow I'm going to try it with two AA cells. The light output is a bit less but should still be acceptable, and two AA NiMHs have nearly twice the watt hour capacity of a single 14500, so it should have plenty of run time.
I'm not totally happy with using flashlights as headlights. From MY point of view, sure, they're great. But they have no cutoff and in my testing this little flashlight gave more glare to oncoming drivers as both low beams of a car. I think I may wind up going with an Ixon IQ primary and using this flashlight as a backup.
BTW, I did some beam comparisons last night, click here to view.
My 3-year-old, 13W TrailTech HID and the UltraFire with all 3 battery options.