The people that I know who have one use them mostly so that they can hydrate without having to stop to pull a bottle out of a cage.

I have one but do not use it -- partly out of fear of disease. I have found that when I used sports drinks (like G2) in my water bottles that they tended to get an accumulation of fungus in them (even though I rinsed them after each ride). When I switched to strictly water in them the problem stopped.

Also, when I bought the back pack, the manager of my bike shop (who has a lot of experience) said that he keeps his bladder in the refrigerator -- not keep the liquid cool -- but to reduce bacterial and fungus build ups in the bladder.

With water bottles, you can scrub them pretty well with a brush. I don't know how you would do that with a bladder.