VERY handy for carrying a quantity of water. Very much better than the 1 or 2 quart canteens. The packs make it all the better. There are several brands but I like the original Camelback. I've got a Mule which is about right for a day's fun. Virginia in the summer can require you drink a lot of water.

These things are all the rage in the US military and you'll see at least one in every in-the-field military news shot. We even issue them now and have a regulation covering the acceptable colors/patterns for those purchased by individual soldiers. Camelback even makes military models to conform.