I don't care for things on my back, and I might be too old-school for a camelback, though many swear by them. I throw extra bottles into a single pannier that most of the time makes the trip, for clothes, lunch extra tire, lock, would not take a pannier just for water, but there is usually other stuff.
A camelback's purpose is not as much for water carrying capacity, which as pointed out can be resolved in a variety of ways, its purpose is to take small sips of water often. I know that when I am riding I cannot be bothered with small sips, I go for large gulps every half hour or so, I manage to stay hydrated, but I admit it is not the best technique. Perhaps with a camelback I would drink more often.