To those of you who use helmet lights as taillights, my observation is that unless you also have a lower-positioned light, it is not so good. Many times I saw people with only helmet lights, the light is so high that it totally disconnects with the person under it. The light often looked like something from afar, then you see the cyclist, and realize: aha, that comes from his helmet! Drivers may not try to look so high for cyclists: the light is not before drivers' eyes.