For those uninterested or unconvinced of the zenlike earth-moving manmachine fusion, fixed gear riding also improves spin. It's an old roadie training technique to spin a fix in the winter to increase stroke smoothness, cadence, and muscle suppleness.

There are lots of other good reasons too.

It's kind of hard to appreciate it until you try it. It's goofy in a completely irrational way that makes you smile or laugh the first time you try it out.

http://www.sheldonbrown.com/fixed.html