Not sure what is is with single speeds, some guys take to them like religion, and other riders go with the gears. A lot of bike messenger riders are into them. My buddy was riding fixed, and after using his ride two times or so, it just seemed...I dunno, logical or something. The quiet is what first appealed to me, and then like just having to rely on your own power to meet the steepness your're riding on. Downgrades without a freewheel can get kind of wild, and mine has a brake. I also like the simple design of a single speed. Check out the velodrome races if you ever get a chance...damn kickazz.
One thing I really like about cycling is the total self sufficiency part....like riding way out into the boonies, and then having just your own power to get your ass home...blown out tired or whatever. I like the challenge in that, so maybe with a single gear it's somewhat more so. Anyway, that's just me, and I respect derailer bikes, even have an old Italian Masi. If I was riding on very steep terrain a lot, I'd go to a three speed.
Anyway, not wishing to change anybody's riding style, talking bikes just gets me going. All bikes rock.