For someone with a big thirst, a very worthwhile investment. No need to constantly stop to retrieve water bottles. Also great because the weight of the water can be carried very close to the body, which is more ergonomic than drinking bottles in the side pockets. The water usually stays cold or no warmer than lukewarm inside the backpack. It's a bit of a hassle when the filled bag has to be stuffed into an already filled backpack. My Deuter backpack also doesn't seem to have an attachment strong enough to hold the full hydration bag in place - it slides all the way down into the backpack.
