I received my Nathan #020 Hydration Vest last month. Here is a long-delayed initial reaction.
First impression last night is excellent. The straps are made of an a reinforced mesh and the edges are covered by an elastic gross grain ribbon-like fabric. The backpack portion of the vest is an oval shaped pack that sits between the shoulder blade points and goes to the top or middle of my low back.
The side harness straps that control the shoulder straps appear well-placed. There are two sets of straps on each side. One set attaches directly to the backside edge of the pack. The second set attaches to Nathan's motion harness. Essentially, think of a boomerang whose middle point is facing down and is attached to the pack. The other two points are attached to the side straps, and the harness itself is made of a stretchy elastic that appears to act as a shock absorber and bounce controller. There is an elastic cord that "V's" on the outside of the pack to hold extraneous gear. This cord is equipped with a toggle and can be pulled one-handed.
The pack has a good amount of storage, even when the 70 oz bladder is full. This morning, I fit my running shorts, long sleeve Capilene top inside the backpack pocket, my shoes in the elastic, my stick of Body Glide in the shoulder strap zipper pocket and a bag of Gorp in the open pocket. I also fit my shoes underneath the extrenal elastic. There is also an outside zipper pocket out the top outside of the pack. This has a small mesh pocket inside it and a clip for keys. I fit my running hat in it quite nicely.
Back to the straps, briefly. They are not a typical ladder lock-type buckle or side-release buckle. It is a simple O-ring that is controlled by a tri-glide buckle. This is difficult to adjust on the fly, and requires adjustment before the vest is put on. That said, the buckle system appears to be solid and designed to hold up to the bouncing and stress of running long distances.
The idea of the vest is to carry an electrolyte mix in a hand bottle, such as Gatorade, Cytomax or Pedialyte, and water in the bladder. This should help me boost hydration on long runs and keep me a happier runner.
I could have opted to get an #028 vest, which lacks a bladder and is also quite cheaper. I opted against it because after contacting the folks at ZombieRunner, I believe the bladder could be easily removed and the pack would carry just fine.
Although I have reserved taking it out for longer runs (10+ miles), my initial impression is that the pack does two things: First, it keeps me incredibly hydrated. Second, it causes me to lean back slightly. This pushes me back on my pelvis and until I have more experience with it, has caused me to run slightly slower. I believe this is caused purely by inexperience with the pack, which will be alleviated with more use.