My main problem with riding single-speed is that the roads are very hilly around here, and there is no way to warm up before encountering big climbs. If I could warm up for 15-20 minutes before grinding up the hills, I don't think it would bother my knees so much, but that's not possible around here. With my single-speed, I found that I had to attack the hills and then spin it out on the descents. My knees did not take kindly to that. However, I enjoyed riding single speed and would probably continue doing it if I lived in a flatter area.