Efficiency wise they aren't any better than toeclips and straps properly tightened down onto old-fashioned cleated shoes, but they're one heck of a lot safer because you actually can get your foot out when you need to without reaching down and undoing the strap first.
And more safely still, when you and your bike are perfoming cartwheels after riding over some unexpected obstruction, like the rider in front, you will come unclipped more or less spontaneously, and won't end up tangled up with your bike in a manner which tends to twist and break bits of you; pictures of the aftermath of the average bunch stackup 30 years ago and now will show the difference.
Second time was when a car pulled out on me as I was going round a roundabout and T-boned my rear wheel. I didn't consciously move my feet to unclip, but I found myself on the road not attached to the bike. I guess they detach under large force, like ski bindings.
Similar thing happened to me earlier this year. Whacked from behind entering a roundabout, Bike and I thrown into the air with me still cleated in as far as I am aware. When I landed I was no longer attached to the bike so I must have become unclipped whilst performing the "spectacular aerobatics" (Witnesses words not mine). Good job we did separate, the bike was written off when the car ran over it just after knocking me off. [1]
Terry Duckmanton.
[1] Insurance company coughed up, got a nice shiny new one now.
