I've been using the B & M Luxos U for about a year now, and very much like how the charging circuitry and battery is integrated directly with the light, and the light itself is one of the best headlight beams currently on the market.

It is designed to charge up the battery enough before charging any device so that the output is always at a constant USB voltage (5v). I have had such great results using it to charge my GPS and phone that I ended up getting a second one for another bike. On the newer unit, the cable to the USB port is fixed to the headlight unit, so water getting into the connection during a heavy rain is not an issue.