I happened upon this great review of the most popular tail lights on the market. The models range from a $3 Amazon special to the top of the wattage scale Cygolite Hotshot. If you're in the market for a tail light. This will tell you all you need to know!
As for my tail light, I have a Magicshine MJ-818 without the battery pack. I have purchased a 18650 battery holder for it, but it hasn't arrived yet. The Magicshine MJ-818 is a 3W tail light with a variable strobe pattern. It's intensely bright but does suck up a lot of power. I have a set of 4 Trustfire 2500mAh 18650 cells which should provide lots (5000mAh) of power to the light. IF and WHEN it arrives!