Last year when I crashed my road bike, I somehow came unclipped from my Speedplay Frogs. I know this because my bike slid further away from me.

One thing I always liked about Speedplays is the ease at clipping out. One thing I don't like is that I was pulling out of them at important points mountain biking. I am now trying some Crank Brothers pedals. I have gone down and still been clipped in to them. They are more difficult clipping in/out; however, I have not pulled out of them when I needed them.