I'm another one in favour of back pack type hydration systems. I just tend not to drink enough water using only bottles, when I get in the groove on the road I don't like to spoil it by sitting up to drink out of a bottle.

As others have said, as well, I use it to carry a folding tire, wallet and food when I'm on the road bike.

I've got one of the smaller ones, but it's adequate, and has the type of bladder that has a large opening so it's easy to fill and clean out.