I've got a Timbuk2 bag, had it for about 7 years now. I don't wear it as a bag on my back, I strap it onto my back rack with bungee cords. It's a great bag. There's a pocket and zipper in front that I put my keys and wallet in. It's a medium sized one, big enough to take a change of clothes and my lunch. It's held up pretty well, and has rubber on the inside to keep the rain out (or in, depending on what you need it for.)

I dont' use it on my back cause it kept slipping around to the front. And i think those bags are built for guys, if you know what I mean.