I also use the FlashFlo Camelbak waistpack. I like it better than having something on my shoulders and back. I never even notice it on a ride, in part because it doesn't hold as much water, so doesn't weigh me down. It's best for shorter rides (up to 2 hours) since it doesn't hold as much, but you can also supplement it with water bottles. On a really long ride, I'll use water in the Camelbak and Accelerade in a water bottle (or two).

Emily