I guess if you want the convenience of using a light with cells integrated in the body then PDW has at least two good tail lights.

But after using lights with higher power battery packs - I don't see big benefits. A MS tail light can be had around $30 and connected to a 4xAA $2 battery carrier from Radio Shack and still out perform these lights while lasting much longer as well.

Of course one could argue the hassle of strapping on the battery carrier and taping or soldering two wires......... In any event I'll take four.........