Today was my first single-speed "half-century." I did fairly well, though I was tired near the end, especially after a long hill climb. I was ahead of the pack of Seattle Bicycle Club riders a few times. On the flats we were doing around 18mph. I can't spin the cranks enough for more than 19-20mph, and so in downhill situations I do get dropped. Still, I find myself climbing a lot faster than I used to, having to keep up my inertia to stop from stalling.
It's a bit of a surprise to other riders that I ride single speed. I was asked "So, do you have a real bicycle?" by one rider who had a carbon fiber bicycle and Dura-Ace components, a high-end wheel set, and color-coordinated bike kit. It's interesting that a lot of people think this way about bicycles without gearing: I did not buy a toy!