I have used a backpack so far on my road bike. I don't mind it so much when my load is light and the air is cool. If I have to carry more stuff the weight is tiring on my back/shoulders. If its warm I hate the extra sweaty back. I do agree with the posters who said it feels great when you ride on the weekend without your backpack, but I would rather have this great feeling every ride.

My new commuter will have a rack, and I expect to use a pannier most of the time. When I decide to ride my old bike, it will be the backpack.