i just picked up a Sealline Urban backpack the small size. Its really a great bag it holds everything i could ever possibly want to carry with me while on a bike and room for some extra stuff if needed. The one HUGE pocket design does kind of suck but can easily be fixed with the organizer sleeve or making something yourself. I havent used it enough to give it a full review but i have taken it with me on a ride and it wasnt uncomfortable at all. It was packed with tools and everything and i barely felt the weight on my shoulders or back. My back did still get sweaty but really thats gunna happen no matter what when something is resting on your back or not. Overall im very pleased with it and glad i waited to find it for cheap. Ended up getting it off ebay brand new for 60 bucks.

Hope this helps.

Ive also looked at the Ortleib not sure if it was the velocity or messenger but they look really awesome and would probably have been my second choice.