Fixed gear mountain biking is like regular mountain biking, but harder. It greatly expands one's likelihood of hitting feet on rocks, roots, etc. and greatly reduces one's ability to reposition weight on the bicycle because one has to keep the pedals moving. Don't get me wrong it's fun, but it's harder.
I've opined about this before, so I won't blather on anymore. However, I did want to lend my support behind the superiority of fixed gear mountain bikes when commuting in the snow. Sheldon Brown knew what he was talking about. 32:19 might be a crappy gear for bike commuting in general because of all the spinning out, but it's great in the snow. Braking without braking is great!
In other news, I wonder how it'll be until mountain biking is possible again.