I have been thinking lately of switching from backpack to panniers, for the comfort, but I still like the original reason I went with a backpack. A backpack allows me to commute on any bike because not all of them have racks, or can take racks. Also, I sometimes stop along the way (grocery store, bank) and the backpack is easier to take with me because it already is with me. I probably will stick with the backpack because, after analyzing all the factors related to me personally, the backpack is the best fit. Not perfect, just the best of all the alternatives.