I only use the Camelbak if I am going where there is no place to refill the bottles. It gets hot here and I sweat a lot so I weigh myself before and after long rides to help gauge fluid loss.
I try to drink a bottle (24oz) per hour at least on hot days and have drank in excess of 200 ozs and still lost 5 or 6 pounds on a ride. One time I tried to conserve my water and came home 11 pounds lighter. Felt crappy for days after that.