I think just focusing on caffeine content is a mistake as well. There are other stims in energy drinks that can crank you up in addition to that.
They are starting to find that Ginkgo biloba could cause cancer so be wary of that.
The best time for me to take a 5 hour energy is before I go out at night with my friends. That extra B12 really helps me prevent a hangover it seems like (as long as I dont overdo it of course).