I bought one because the size listed indicated it would fit in the saddlebag of my motorcycle. I also liked the fact that the lens cases could be removed. The measurements did not take into account the hip padding so it doesn't fit. Carried it around Las Vegas for a week and on a cruise to Alaska. The bag seems to be pretty big for a beltpack. Used it that way in Las Vegas (no lens cases) and it was pretty bulky, and quickly went back to using the shoulder strap. The belt stays tucked in between the padding and the bag now. In Vegas I was only carrying the 20d with a Canon 28-135 lens.
I liked being able to take the lens cases off and only carry what I wanted for the shore excursions on the cruise. I also usually forget which case has which lens, and therefore end up opening up the wrong one first. Not so bad but something that wouldn't occur with a normal shoulder bag.
I was able to fit a 20d with the 18-55 kit lens, Canon 28-135, Canon 100-300, lens hoods, 2 spare batteries, and the camera strap in the bag with the 2 lens cases. Not sure if I could also get a flash in there too. I packed the battery chargers and off load device in my regular luggage.
Overall I like the ability to scale it down to what I need. As a bag to carry everything it falls somewhat short.