I really like the Novara bike commuter backpack from REI. There's a chest strap as well as a waist strap to hold it firmly in place. It has three different zippered compartments plus a handy place to stuff your bike lock. You can really pack it, almost too full to carry. It's bright yellow and has lots of reflective bits, a place to clip a blinkie light and good padding against your back. I use it when I ride my road bike to work.

When I ride my trusty Surly I use panniers. I really like them too. I use my Ortlieb front panniers on my back rack. They are the perfect size for day to day commuting, plus waterPROOF. That's important for my foul weather commutes. I don't feel like stuff gets too jostled in the panniers. They are usually stuffed fairly tight with "stuff". I don't seem to be able to travel light. Panniers are a good choice if you have any back issues. Plus, being the moist old bird that I am, my rain gear vents sooo much better with out the backpack. bikerHen