Fixed mtb-ing is a blast! I ride fixed on all the technical stuff I ride on my geared/ss mtb's. It's not a whole lot harder, just a different game.

And +1 on using a brake or two. I only use 1, have never needed more than just backpressure on the rear or hand-brake in the front.

Here's a picture of my current fg-mtb, my '99 Gary Fisher Kaitai:




And here's a pic of my fg-cx bike I raced on a couple of seasons ago (actually placed very well with the right gear choice):




Another shot of the fixed mtb, different parts on it, taking a break on a ride: