I have a cheapy Karrimor bag that I run to work in it carries a change of clothes and my lunch. It would be able to carry a bladder I think too. The only real problem I have with it is the straps rub on my neck but I think that would be solved by homemade covers when I get around to it. It is very, very light which is a big plus for me. I like the pockets on the straps too.