The problem with carrying a backpack is that it necessitates carrying more stuff like a change of shirts because of the extra sweat, etc. That said, I love my Osprey bag because of the support mesh that helps keep the back cool. During graduate school, I commuted every day to school and to hospital internships, so I couldn't show up sweaty. I had a small cabinet under the desk where I could keep my backpack, helmet and shoes, so it worked out fine. I'm in the process of moving to my next clinical job which will be about 8 miles farther away and I'm not going to be wearing clipless shoes, so I have to be looking at other options. Anyway, I digress. The messenger bag thing might be cool, but it's for folks who need quick access to their cargo. You probably don't want all that weight on just one shoulder and a little support from a dogleg strap; that said, my Crumpler messenger bag got me completely through undergrad and years of bicycle wrenching, and it still is in fantastic condition, albeit a bit dirty. If you must go with a pack to be worn on your body, get one that has good breathing room on your back and can accommodate hip support. Good warranty and customer service are pluses.