Fixie off road riding is my very experience is quite scary. I havent got use to the lack of back pedal yet. I think the trick is to plan the trail in front and be mindful of any tree stumps that the pedal might it.
But on the road it is fine, the only thing that was weird is starting off and the pedal was in the wrong position and you can't pull the pedal around backwards. But after a couple of rides you soon get use to it. The other weird thing is riding backwards, I'm sure it will look cool when i have mastered it!