I have the usb front light. I originally bought it because of it's small size and the velcro strap which fits on my 31.8mm bars AND on my smaller bullhorns on my other bike. In L.A. there's enough street lights so that I don't really "need" a light for seeing the road, but it's bright enough on high (there's two steady light settings) to see the road on dark streets if i'm not going too fast.

The only downsides is I have to charge it almost daily to make sure it's always got power for my both my rides (morning/night), and the other downside is if it does die, there's no warning. It just turns off. I assume the back is the same.. not sure. I always carry a backup light in my backpack, but haven't had to use it in a while.