I have a rack with shopping bag panniers. It's like ragging a parachute. I can haul my clothes, computer and lunch as well as a few student papers, but it makes the ride slow.
On light days I carry a backpack. I roll about 12 with the panniers, 15-16 with the backpack. Huge difference in feel and fun. Still, on days i need to haul, its the panniers. They are also nice in teh heat,, when a backpack feels pretty hot.