I purchased the Camelback Alterra hydration system to take on my longer marathon training runs. I chose the fanny pack because other Camelback models sit across your back and can make you overheat.
The fanny pack was fairly comfortable. The weight of the water is evenly distributed across your hips and lower back and the wide belt allows you to tighten it enough so that it doesn't jostle, yet remain comfortable.
Having the water valve right at your hip is nice, because you are not fumbling with shoving bottles in and out of holsters while trying to run.
You can easily fit power gels, ibuprofen or an ipod in the pockets on the Camelback.
It only holds 28 ounces of water, which isn't much on a three hour run.
It is difficult to tell how much water you have left when using the Camelback. I found that sometimes I would drink it all before knowing I was nearing the bottom, and other times I would be so afraid of drinking it all that I would not drink enough.
It is also very difficult to clean the bladder inside, and dry the hose and mouthpiece.
Overall I would recommend this hydration system to carry on one to two hour runs. The volume simply isn't large enough to last longer, and you are better off stashing water on your route.