I'm buying a Chrome Ranchero backpack in 2 months, and right now I use a Targus laptop backpack.
However, I have a rack on my bike, and I also have a trunk bag that I use. I think it's a good idea to have both, as it's no fun to have to dismount the pannier every stop when you're only carrying a paperback and a few other things with you. That said, when I have a load of binders and/or textbooks to bring someplace I use the rack because I have scoliosis.

In short, backpacks are great for smaller loads and just cruising around with the necessities, and racks are to be used for the hauling.