I recently built up my first single speed (since my first grade BMX) as an in-town commuter/snow bike/project.
I was actually rather shocked by the feeling of efficiency and immediacy in pedaling. I had to get out my multi speed bike and put it in the same gear combo just to make sure I wasn't just imagining the difference. Has anyone figured out the mechanical advantage in having a straight chain line vs. the more convoluted path of a derailleur chain? I still reach for the shifter as a reflex now and then, but I suppose that it will soon pass.