Lots of commuters swear by single speeds and fixies. I had one built up a couple years ago for occasional commuting, as a change of pace. It looked great and was a blast to ride, but it did a number on my knees. The roads are pretty hilly where I live and it is often chilly or cold during my morning commute, and I wasn't able to warm up properly on the SS before hitting the hills. Basically you have to charge all the hills on a SS/fixie, and that didn't agree with my knees. I started developing knees pains almost immediately, which I stupidly ignored and didn't attribute to the SS, until the pain got so bad that I ended up in physical therapy and missed two months of riding. I put the gears back on that bike and sold all my SS/fixie parts.