I can't add much to the above responses except a +1 -- fixed-gear riding has made me a much stronger rider in general and has helped to break some bad habits. In theory, I could have accomplished that on my road bikes, but a FG forces you to work harder on the uphills and to stop coasting so much.

I haven't done a fixed-gear century yet, but I've done several FG metrics, and the itch is there...