I've been riding clipless for several years and I still have the occassional "failure to unclip" incident in which I topple over like the little old man on the trike on Laugh In. Don't know whether my mind throws itself into a feed back loop and can't decide which side to unclip, or if the foot sometimes sticks in the clip or my subconcious suddenly decides that it is time to attemp a track stand for the first time ever, or whatever. Fortunately my hips are quite padded so it's usually more embarrasement of falling over in front of a bunch of cars or riders and car than resultant injury.

All that notwithstanding, I really believe that the SPDs are responsible for the smoothness and efficiency of my pedal stroke, which translates into more mileage endurance, and strong but not terribly bulky thighs since I am using the muscles in my leg as much to pull up as to push down.

Just saying I love the clips, but I also have double sided pedals which allow me to ride unclipped which is good for crowded group rides, approaching intersections, uncertain road conditions etc.