Timbuk2 has horrible strap designs. They're only decent as shoulder bags or for carrying light, flexible loads on an upright bike. In the drops, heavily loaded, on a road bike, they're absolutely awful.

I own a PAC Designs Ultimate which I regularly use to haul 20 - 30 lbs back and forth from work, in addition to hauling 40 - 50 lbs of groceries. The bag is absolutely amazing, but will run you around $400 with shipping from Canada. They are generally regarded as the best messenger bag on the planet, and you pay for that.

I've heard good stuff about Seagull too.

For hauling school-type loads, unless you want to buy a PAC along with the X-strap and waist belt (more money...), I'd get a Chrome Sultan backpack. Big, comfy, well made, lasts forever, and honestly, Chrome is the best bang for your buck.

Chicago Wig also makes a backpack, and they seem to have a good reputation, in addition to Reload and Timbuk2 (their backpack seems good, even if their mess bag are not).

Most people would just use panniers for hauling school books though. I don't like panniers, but I can imagine using them if my daily load was 40 - 50 lbs.