I've been riding clipless for almost three years. Once, I put some straps and clips on my fixed gear and went for a ride. Quite scary. I had lots of trouble getting in and tightening them. Then I had trouble getting out. After one trip around the block and put the clipless pedals back on.

Also, I've never come out of clipless other than once when I was trying to majorly mash the big huge gears on my road bike. And I keep them adjusted pretty loose.