I have several camelbaks and I use them often for long runs, hikes, etc., but I don't find them comfortable for riding (and I tried using a camelbak a few times before I was comfortable reaching for my bottles). I never adopted it because it was hot and, while I didn't feel like it threw my center of gravity off, for some reason on my road bike it caused me to tense up my shoulders/neck more than I ordinarily would, leading to discomfort. I fully own that this was my individual problem and may not apply to another soul in the world -- but I put it out there to defend the viewpoint that there are legitimate reasons NOT to use a camelbak for some people. Guess what -- through a summer of hot rides, I learned to use my bottles while riding at speed. What do they say... necessity is the mother of invention ...
I also had neck/shoulder problems with the camelback when riding my road bike.
Never had a problem on the mountain bike, due to the more upright riding position.