I built myself a singlespeed bike last November. It was a fun project to make use of a frame that a mate had donated to me. I already had spare wheels, tyres, bars etc so it only cost me about £70.
TBH, I already had a mountain bike, a cyclocross bike and a second-best road bike so I didn't think that the singlespeed bike would get much use but I really like riding it. In fact, it looks like I will have notched up
at least 2,000 km (1,250 miles) on it by the end of its first year including several long rides (metric centuries, imperial centuries, and one 200 km ride). It has certainly become my second-best bike in terms of usage.