Hydration packs are great under certain conditions. I always use use one of my Nathan vests in ultras where I will be using trekking poles. I just can't figure out a way to combine hand held bottles and poles. They don't bother me at all and I don't seem to have a problem with the packs rubbing sores on my back as I understand some do. They are also great for storing all of your supplies. This is really important if you are running in uncertain weather conditions or in the mountains where the weather can always be questionable.
In all the 100 milers I have run I wore one of my hydration packs, even if I was carrying water bottles. The pack (without the bladder) gave me room for things like spare a flashlight and batteries, light rain shell, a place to store my hat and gloves after I took them off as the temperatures warmed, trail directions or map in a baggie, a little toilet paper in a baggie and a few other things I did not want to be without.
I have a Nathan Endurance 70oz pack
and a Nathan Xtreme 100 oz pack.
The Nathan Quick Draws I carry most of the time have two major disadvantages. First, as I mentioned above, you hands are not free or at least one hand is not free.
I tried the Amphipod Bottle, mostly because it came with a neoprene sleeve, but I could not keep the bottle strap together during runs. The top of the strap fits around the neck of the bottle and bottom strap is hold in place by tension between the two. Loosen the tension and the bottom strap (loop that fits around the bottom of the bottle) just falls off. To keep the strap attached securely, the part that goes over you hand must be tight. That bothered me so I had to loosen it a bit. When I would take my hand out at an aid station to refill the bottle, everything fell apart. I retired the bottle.
That did give me the idea to put neoprene on my bottles. First, I used the one that came with the Amphipod bottle and it really helped with the freezing hands but I could not tell how much water I had left. Then I added my own layer of rubber and have used that ever since.
The following is the original article as it appeared in 2011. I have made several changes in how I handle electrolytes while running now, but I am writing a new article on Hydration and Electrolytes that I will post next.
Like just about everything else involved in ultrarunning, it depends on what you like and the race you are running. Personally, I prefer the hand held bottles like the Nathan Quickdraw. Here is a link to Nathan hydration products. I used them in the first two 100 mile races I ran and they worked great. I use them for all the 50Ks I run and every training run I do that is less than 5 hours. I also use one in every road race I do including the run segment of Ironman events. You can even carry a flashlight while running with them.