They're easy to take on & off, and easy to swing around to the side/front to access the contents; they hold large & oddly-shaped items; they are more easily loaded & unloaded due to the large opening; they keep stuff out of the weather better than *most* conventional backpacks; they make the wearer feel just as happy inside as he/she is trendy on the outside.

I use one, and I like it 'cause I can fit lots of stuff inside it. I'm not a bike messenger. Truth be told, I just want to reap the benefits of looking like a member of a fringe group. I tried to fit in with the Latin Kings last week, but they scorned me, just like the Professional Association of Mimes did back in January. It was amazing how quickly they confiscated my striped t-shirt, my suspenders, and my beret, without speaking a word. Didn't let me out of the invisible box for like 2 hours!!111!11