I second or third the RELoad call. I'm not a messenger, but I commute daily with laptop, digital slr and lenses. It has room enough for all my toys, cables, and tools plus a full change of clothes for the office, a towel, and lunch. I've got room left over for lots of books. And all this without having to decompress the compression straps, which also double nicely as a place to put a mid-sized tripod.

It has a metal buckle. I went from an all corroded to hell DeRossi (sp?) with metal straps to a Timbuktu with plastic buckle. Some people are really into the Timbuktu buckle, but I found it often disengaged at the wrong time wearing it high on the back. When I replaced it with the RELoad, I felt like I was home again. Something to consider, especially if you're going to be using it as your primary bag.