Has anyone here ever used the Camel Bak Hydration system backpack?

I know that it is used as an easy and constant source of drinking water. What I'd like to know about the backpack is if it also has other uses? Would there be space to store several other small (survival related) items.

Also, about how much water is carried in the reservoir? Is there about a full day's supply, or would you have to carry extra water to refill it within a day's time?

Thanks!