I always use one on my mountain bike unless I'm just going out for a short jaunt and will take one bottle with me instead. I also wear one on my road bike although all the roadie purists will tell you that is just not done! But, I have small frame bikes (both mountain and road) and one bottle is all that will fit on the bikes comfortably and one bottle just is not enough. I use a small light weight camelback on the road most of the time and a larger heavier one off road (cause I live in the mountains, need to be self sufficient and be ready for monsoon afternoon thunderstorms, i.e. carry more)

Some people I talk to hate camelbacks cause they don't like to carry the weight on the shoulders, the will complain about neck pain, back pain, etc., but I find that if I don't overload too much, I'm fine. All of these things are personal preference, just like bike seats, etc. But you asked so I'm saying Yes to camelbacks.