I've ridden my Monocog 26r off road fixed probably less than 20 times and I'm not a fan. I just found it a lot less enjoyable. Unless you get the gearing perfect it's nearly impossible to go fast enough to enjoy. Climbing is the same, but rolling hillly sections and descents suckass. For it to work for me, I would have to run a much larger chainring and switch it when I rode fixed. I will probably still play with it for fun, great wheelies.
So, IMO Single Speed MTB is way more fun!