I'm sold on using a Camelback. I've been using one while riding my single road bike for many years. I also started using it while single mountain biking a while back and I find that I drink much more. The mountain bike certainly demands alot of attention from my hands and the camelback does not. I spent this past winter and spring using regular water bottles on my road tandem, and I found that the early warmth that we had snuck up on me. I wasn't drinking nearly enough and I felt it on those unseasonably warm days! I will never again use water bottles except as storage from which to refill my camel. I don't like the weight of 70 or more ounces of water on my back, so I refill often. I am trying to convince my wife (stoker) to use a camel back, but she insists that she finds time to drink enough with conventional bottles. The fact is that I am constantly reminding her to drink.
I've done a few mountain bike trips in the Utah desert, and I'm sure that the almost total lack of humidity out there would have really hurt me if I didn't use a camelback.