I used to use them a lot in adventure racing.

These really only work if you put enough water in there to make it drinkable(you don't want to dilute it too much though).

I just bought this cool waterbottle at the Sea Otter. Its called the Gel Bot and its a 20oz. water bottle with a seperate chamber that holds gel. I haven't tried it yet but it seems like a neat idea. http://www.gel-bot.com/