I've been commuting by bike the last month. I have two bikes which I use, one is a single speed the other a track bike. The single speed is a KHS Flight 100 which used to be my fixed gear bike. It was the only bike I had that was set up to take a rear rack. I installed a rack so I could carry panniers on the rear. The bags allow me to carry achange of clothes, lunch, drinks, extra tire and rain gear. With the extra weight on the back and riding in traffic I decided to try it as a single speed with a free wheel. I also opted for platform pedals without clips. A pretty standard single speed set up. The bike works fine as a commuter. The extra weight on the back affects the handling especially in windy conditions. Overall it serves it's purpose. The gearing I think is a 48/16 or 17 which seems to be perfect.
Now the Fixed Gear bike. A classic steel frame track bike. I run a front brake, chopped drop bars and 48/17 gearing. To carry my stuff I have to carry a backpack. The backpack works fine except when I carry my lunch, then it's a tight fit. This set up is definitely a better ride. You don't notice the backpack that much and the bike handles a lot better then with panniers.
Riding the fixed to work is definitely more fun! The bikes faster and handles better!