While the belt fits great and holds the flask securely, the pocket is too small for anything significant. (Read: Cell phone and keys won't fit.) So I find it useless for me and am forced to wear ANOTHER waist pack with this to hold that stuff (man, I miss Nathan's old 10k waist pack).
Worst part, however, is the water bottle. It leaks around the threads and is difficult to close securely, thus you end up with sports drink all over your backside. Not a nice feeling during a 13-mile run!
I'm switching back to just a waist pack with the FuelBelt Wedge Bottle with Clip (found here on Amazon) because the flask is so much better.