What are you going to use it for? (Besides riding at night, I mean!)

For occasional use, I think the Cygolite line has some of the best values. Bright lights, decent optics, reasonable cost, solid mounts, good runtime. I run an older Rover II on one bike (still running), and bought one of their integrated lights, an Expelion 250, for my daughter this fall. She likes hers, too.

For longer rides, or regular commuting, it's hard to beat a dynamo hub and LED light. They're pricy; figure on $200-600 for a complete set. I got a decent outfit for about $300 when velo-orange had a Shimano DH-3N72 wheel available. The main benefit is that it's always there and ready to roll. No batteries to charge or discharge.

My IQ Cyo lights up the road quite well. It seems pretty waterproof, as I got caught toward the end of last summer in the worst downpour I've ever ridden through. The hub and light worked fine, although one of my battery-powered blinkies gave up the ghost. That was a ride I would not have had a light on if I'd ridden the other bike. The battery and light were still off because I was going to be home well before dusk, except that I took shelter until the worst of the lightning had passed, and so I ended up riding home in the dark.