Love it now, but not at first try.
I bought it for marathon training. I already have a Camelbak Catalyst but needed a higher capacity for longer runs (45 oz vs. 28 oz. or 18-20 miles vs 13 miles for me). I'm definitely biased as I prefer Camelbaks over water bottles! The FlashFlo reservoir is way easier to fill and put back into the pack than the Catalyst. The belt tightens in front - another improvement for ease of use. The side-cinch compression system allows one to tighten it as you drink to reduce the annoying sloshing and help keep it from riding around. Two, more than ample zipper pockets for gels, keys, etc. The outer pocket is designed to hold a CD or MP3 player but I don't run with music so I've never tried that feature. It took me two runs to get accustomed to the adjustments. I kept prematurely tightening the side cinches at first which causes the pack to twist upside down. It also took some time to figure out where to let it ride on my back comfortably. I wouldn't recommend it for small people as the filled reservoir weighs nearly 4 pounds. (I weigh nearly 200 lbs.) Once I figured out how to adjust it for my comfort, I really fell in love with it.