My $.02...

I was using a backpack for a while and I found that no matter where it was on my back it was always pulling back my shoulders and made my neck hurt if I was in the drops (sometimes, but rare). Plus, all that tight clocse contact was WAY hot in the New Orleans climate. I bought a discounted crumpler bag to test the mess bag configuration and found that it was much more comfortable and cooler (as in temperature.)

Plus, after I broke my collarbone last year backbacks HURT like hell when my shoulder gets sore. Which is odd, because I thought the backpack would be better.

After I started doing on-site appointments and needed to bring cabling, routers, modems, tools etc with me I had to upgrade to the bigger Jandd Iniki and haven't looked back. Messenger bags are doubleplus good while riding, IMHO.