I have commuted with a back pack and then decided against it. Not because it was too hot, but because it couldn't hold what I needed it to. I opted for a Chrome messenger bag and have not looked back. I don't do the whole basket/rack/pannier/whatever commute. Neither of my fixed gear bikes accept those and that is fine with me. I get to work in the summer, and my back is sweating, but then again, I am sweating as well. I don't dilly dally when it comes to riding to work. Get on the bike and move out! I don't have time to dink around with panniers and adjusting the load and ensuring that it is even. I pack my bag with my necessities and hit the road. It sits nicely on my back, doesn't shift around, and holds everything I need while keeping the lines of my bike clean and my load light.