Originally Posted by slvoid

In the past decade, I've gone through 2 timbuk2 bags, 2 chrome bags, a crumpler bag, and a marmot backpack over... 30-40000 miles of riding? I think one year I rode like 15k, more than my first car.

In my experience, the messenger bags are best if you have to open and close your bag frequently. The chrome bags fit better on the bike while the timbuk2 fits better off. The crumpler bag fit horribly but the back was some really tough material that tore up my jerseys like sandpaper but kept it from moving around too much. Like the others say, they have to be cinched real tight and ride pretty far up your back in order to fit well and be stable. Best bag I ever used was the marmot. It's basically a backpack with a suspension that lifted almost the entire pack off your bag. Once cinched up, it hugged my body and everything stayed tight while at the same time allowing my back to breathe.

Your mileage may vary...