Backpacks can be hot, sweaty, and literally a pain in the back. Went from back pack to rack trunk and now to panniers, the latter change being prompted by the need to carry stuff for evening classes. I love not having the weight on my back and would never go back to a backpack unless my trunk and panniers all but the dust for some reason, and even then only until replacement could be procured.

Then again I now ride a Surly LHT with both front and rear racks so I've got plenty of places to mount panniers.if I were so inclined to ride a lightweight road bike to work I'd see the appeal of the backpack.