My old Nathan HPL 020 is on its last legs, so i bought the Zelos as a replacement. It's a great product, and most of the complaints i had about the 020 have been "fixed" with the Zelos.
Light, comfortable, easy to adjust the fit, and a good amount of storage space. Holds a 2 liter bladder in the back, and comes with an 18 ounce speed bottle that fits in the front right pocket. That will come in handy for longer runs in the hot weather. There's room in the left front pocket for another bottle if you wanted to carry more hydration. Drinking out of the tube is simple, and much easier than with my 020.
The only thing i don't care for is that the included safety whistle (a nice touch) is attached by a cord, and it bounces around (right in the face!) as you are running. That issue was easily solved by stuffing the whistle in the pill pocket, and when i need to start carrying electrolyte capsules i'll probably just detach the cord/whistle, and stow it in one of the other front pockets.
There are a lot of straps to adjust the fit, as well as a couple for cinching down the pack. The external shock cord system is much improved over the 020, and will hold whatever you put there securely.
If you're looking for a new hydration pack, definitely check this one out.
1- the safety whistle issue isn't a problem after all, as i discovered it has a built in clip to clip it on to one of the straps
2- i had a problem the first few times using the pack with the speed bottle bouncing around too much, but i realized that unlike my previous pack, the sternum strap on the Zelos can be adjusted up or down to whatever location is best for the individual runner. I just moved it down a bit, and haven't had a problem with the bottle since.
I can't wait to hit the trails this spring with this pack!