I think there's a bias towards messenger bags because they're "in" and not necessarily because they offer the best function....but then again I do agree that the center of gravity and less sweat thing can be nice. My problem was that by the time I put in lunch, laptop, and/or clothes for the day it got too big and heavy. Not too bad on the bike, though even with the Timbuk 2 strap once in a while it would want to swing around and mess with my balance. But I developed some shoulder pain and finger numbness from carrying it around and that was the end of that. I've kept it around for lighter/smaller loads because it's easy on and off and fun to have. I prefer panniers for most loads by far, and don't worry about the wider profile. However, for my current bike - bus - bike commute I found the panniers didn't work as well so I usually go with a lower riding backpack. I second the notion of keeping the bags as small as possible - it's too tempting to fill them up!