I love my Planet Bike Alias. The basic 10W version is available for less than $100, you can get the 15W version for a little more. I actually started with the 10W version and then upgraded it. Most of the other light systems i've tried, cost more and didn't last as long. Most only lasted a year for me, either the rechargeable batteries stopped charging, or the wiring corroded. The Alias has lasted me 3+ years. My only gripe is that I have one bike with handlebars that are thinner than the clamp will work on. So I wrapped 1 layer of duct tape around my clamp locale, and voila it clamps.

I've put the thing through H E double hockey sticks. rain, snow, hot, cold (less run time in the winter though, as with all batteries, but I still get the 1 hour for getting home). I even tried strapping the battery pack to my downtube, and the thing got sucked into my wheel. The tire rubbed through the thick plastic housing, right down to the battery, but it still works, bare metal battery side exposed and all. Amazingly even though I decelerated from about 35-40 (goin downhill) to zero in about 2 seconds, I did not crash.