I converted a beat up Schwinn LeTour to a single speed for riding around my relatively flat city. It's a nice change and the maintenance is minimal. It's also lighter to carry up and down steps into my apartment, and less to worry about with locking.

For more serious rides I'll break out my other bikes, but for general grocery store runs and the like I'll take out the single speed. The ratio is pretty small (can't remember off the top of my head) so it's not the fastest ride, but the direct chainline is smooth, and the slower pace is pretty relaxing.