I always ride clipped in, but this is more to do with the fact that the last time I rode a bike that I wasn't mechanically attached to was in 1977. The common wisdom amongst the best riders seems to be that it is better to learn to ride properly off road with flats, and then when you move to clipless you have the benefit of the clipless pedals, but the correct skills, I plan to get some flats and spend a few months riding with them, as this should make me a better rider, however, clipless have become my comfort blanket and I'm struggling to think about riding without them, definately make sure I've got some shin pads on when I first try the flats. I never have a problem unclipping, I think it does become second nature and very natural after a while, and I do ride some very technical rocky, rooty, steep stuff, at the weekend I road some very steep hidden trails at Innerleithen (too steep to walk up type stuff) and as cubist mentioned in similar situations, getting going after stopping can a bit tricky, but there were lots of trees to hold onto as I clipped in. If you just want to enjoy your riding and have fun on normal trails, learn to ride with the clipless and stick with them until you have no concerns, if you want to take on more serious stuff, have a go with flats just to help your skills along.