I've never actually used a Fuel-Belt, but I love my Amphipod belt. I have one like this (20oz bottle):


During marathon training in the Summer I would use that bottle for sports drink, then on the front of the belt I had 2 smaller (10 oz.) bottles that clip on and off that I'd have filled with water:


If I liked gels I would have probably tried one bottle with gel, since that's cheaper and less messy than the packets. But I prefer sport beans, so never needed the really small bottles.

Since I have relatively few places to refill my bottles on most of my running routes I never had any interest in the Fuel-Belt system...I would be dehydrated if I had to rely on such small bottles for fluids.