I must tell you about the greatest hydration pack ever: the Nathan HPL 20 Hydration Vest. (They didn't sponsor or provide anything for this review by the way.)
This was the third hydration pack that I have used. The first was a Costco special that was fine for hiking but totally uncomfortable for running. The second was the Camelbak Rogue. It was more comfortable than the first but still not great. It rubbed against my lower neck and bounced around quite a bit. It's not a horrible pack, but not something I wanted to wear for tons of miles.
I noticed that in pictures from ultramarathons, many runners were wearing the Nathan hydration vest. I'm training for my first 100 miler so I figured I should go with the experts and try Nathan. Notice that they call it a hydration vest instead of a hydration pack. The vest wears differently. It's kind of like the vest is hugging your body instead of riding on your back.
This made it super, super comfortable. It doesn't bounce around even when it's filled to the 2-liter capacity. It has some storage on the back, but my favorite part is that it has two storage compartments in front for easy access. I keep my camera on one side and my gels in the other. It is SO nice to not have to take my pack off to reach my camera. My sister described the look perfectly: from a distance it looks like a saggy bra.
*Easy to refill
*2 liter capacity
*Storage compartment in the back
*Two storage pockets in front
*None (if you don't count that saggy bra look)
If you're in the market for a hydration pack (or vest) for long distance running, the Nathan HPL 20 is definitely the way to go.