I ditto the Nathan pack. I have the 2 bottle waist belt and I absolutely do not get bothered by it. I drink a ton, and even on short, 3 mile runs, I finish one of the 10- oz. bottles.If I ever ran longer (above 5-6 miles) distances, I would need the 4 bottle pack.
I have a different waist pack for hiking and x-country skiing, with a larger pouch to store gear. I could never wear it running. Same way I couldn't ski with my Camelbak, although it does not bother me for riding.