I used to use a backpack on my commutes. With a sore lower back and the sweat that came with it I changed over to panniers. While I do find them useful, I feel a rack trunk is the best choice. Otherwise, I tend to haul everything including the kitchen sink. Not to mention the areo hit it caused me. Now I pack what I need for the day and call it good. This is entertaining when you can have a 15F morning and a 60F afternoon in Colorado.
I still use the backpack for around town errands. Panniers are used from time to time when I need to haul something to work.