I used to make fun of people I saw running with these things on. In all honesty they are silly looking. But for long runs they are a life saver. It takes about two minutes of being on my back before I forget it is there. It is much better than carrying a bottle, and holds about four times as much. It supplies enough water for fifteen miles on a hot day. There is a little pocket on the back that can fit your keys and an ipod, which I prefer to having my ipod strapped to my arm. The pocket is not good for gels or anything that you want to access without having to stop and take the pack off. I think the greatest benefit of Camelbaks is the freedom of not have to plan runs around water breaks. For the price, I really can't recommend the HydroBak enough.