I too use clipless (spd) pedals on all my bikes, commuter, mtb and road. I currently have flat one side/spd the other on my commuter hardtail and they're driving me nuts because they keep flipping over when you've clipped out (say at traffic lights) and the spd side is always underneath when I want to get going again....

So I've just bought some mtb double sided spd pedals to swap them over. I use shimano 520's, they're great! And not too expensive either!

When I use the flat pedal side on my commuter pedals, I hate it, my feet forever feel like they're going to slip off. Like most people, it took a while to get used to spd's but once you've mastered it, they are so much better. And...really important...they are much more efficient on the bike as you use the whole cycle stroke, not just the downward one. You'll use muscles you never thought you had

I prefer them for mtb riding as well, but a couple of 'the boys' I ride with swear by flats, its what you're more comfortable with I guess. I had a nasty 'off' last year and twisted the ligaments in my ankle simply because I didn't get my feet out of the pedal quickly enough when I fell, the bike went one way and my ankle the other... But hey, I could have suffered a similar injury by sticking my foot out when I fell anyway!

All I would say is loosen them right off when you first use them (there is a little allen screw that adjusts the tension) and gradually tighten them up as you become more confident. They really are the best way forward I reckon...good luck!