Forgive if there are some redundancies in this post from previous replies...

Camelbaks are not a replacement for canteens or water bottles in all situations, however they have there place and are a great tool.

The major benefit of camelbacks vs. bottles is that the use tends to hydrate more often and more consistently! I have not done a scientific study, but my observations of my students have been that those carrying a camelback drank more and showed less s/s of dehydration than those using bottles of some kind. This reason alone makes bladder systems of huge benefit.

Another benefit is that a much larger capacity of fluids can be comfortably carried in bladders than in bottles. I tend to carry 3L for every 2-3 hour between water supplies, depending on weather. In some places that equals 9L of fluids (yes that is almost 20lbs of water); 3 - 100oz bladders are easier to fit in a pack and carry than 9 nalgene bottles.

When traveling in the back country, I have one “dirty” bladder with me. The dirty bladder is used for passive filtration. I hang this balder high in a tree and attach its hose to the “in” port of my filter and a clean bladder to the “out” port. A few minutes later, no pumping involved, 3L of filtered water.

Another benefit isn't of importance to many, but speed is a big benefit to me. Not having to stop and pull a water bottle from my pack to replace the one on my belt is important to me. I have also found no good way to carry 3L of fluid and a few essentials on me in a setup as quick and light as a Camelbak M.U.L.E. With my bigger packs, I can have two bladders, one with plain water and the second with fuel (Infinity Nutrition or Sustained Energy) – fuel and hydration on the run!

Now, I still always carry a 1L nalgene bottle when on long treks, and over night trips. I typically use it only to carry a little extra water and mix and carry drinks in. And on short runs and rides I still use 20 & 24oz water bottles, but the camelbak is much better (IMO) for long trips.

And yes, you can freeze Camelbak bladders and get cold water out of them… I use two techniques, first is fill the bladder 75% full and let a little air remain in the bladder. Lay bladder perfectly flat in the freezer the night before a trip. Be sure that there is no water in the tube or against the outlet at the base of the bladder. In the AM, add the remaining water. You may need to run a little hot water over the lid to get it off. If you are bringing multiple bladders, you can fill the extras 90%; just make sure there is no air remaining in the bladders. I have done this many, many times with great success.