Here's where I shill for Timbuk2...
Product: Timbuk2 Messenger Bag
Available at: Timbuk2.com
I've had my bag for 9 years. I have the "Deedog" model, from back when they still gave the bags names. Now they just come in S/M/L/XL. It has survived 4 years of all-day, every-day college use and 4 years of civilian life. It's been through rain and snow, it's been dropped in the mud, stepped on, run over by bicycles, and chewed by dogs, and still looks almost brand new. This thing is indestructible. I take it on every plane trip I go on, and every time I do, someone asks me about it. "Dude, cool bag. Where'd you get it?"
Oh, and even after nearly a decade of being loaded with heavy stuff, the strap is still stiff and distributes weight nicely. Fully loaded, the bag still feels light. I've carried books, computers, clothes, even groceries for miles in complete comfort.
And the whole thing is designed to stow securely on your back for bike riding. The length of the main strap can be adjusted, and there's a secondary strap that clips under your left arm to keep everything in place. (Check out their "How to Wear" page for the visual aid.)
After I got my bag, both my roommates were impressed enough to buy their own, and as far as I know, all three of us still use them regularly.
The Timbuk2 website lets you build your own bag, including colors, fabrics, accessories, and whether you want a right- or left-handed bag. (Handedness affects where the adjustment handle for the main strap is.)
I haven't tried any of the accessories they offer, but if I get the Powerbook I'm eyeballing, I might just have to spring for a quilted laptop sleeve.
This bag has not only kept me satisfied for nearly a decade, it's kept me from even considering another bag. If I didn't already have one, I'd be jealous. If, somewhere down the road, this bag gets lost or stolen (I'm convinced it'll never wear out), I'm just going to head right back to Timbuk2 and get another one.