Some observations that apply to several of the posts above.
Toe Clips vs. "clipless": I rode toe clips for many years (decades actually) and even though I was in the bike industry through 1996 and still do repairs and generally keep my "hands in the grease" I did not have clipless pedals till last year. I would never want to go back. I use a touring/mtn type shoe and two sided Crank Bros Candy pedals and love the ability to clip in and out easily and the lack of pain and circulation restriction that I had with toe straps. From the beginning I did not notice any problem getting out of the clipless, even when I remembered late. That is opposed to the first time I rode in toe clips with cleats and forgot to loosen the straps at a stop sign on my first ride. Pride hurt more than body of course. Walking in cleated shoes was always a pain. Besides that I no longer have to worry about toe clips bolts loosening, straps breaking, or buckles not working properly.
Safety: There is no question in my mind that being clipped in offers a lot of safety advantages. I can much more easily accelerate out of harms way, keep my feet on the pedals during wet weather and on bumpy roads and can easily float/jump over obstacles and potholes. It's also next to impossible to catch them pedaling through a turn.
Efficiency: I've seen only 2 or 3 studies on this. I remember one showed greater oxygen consumption with toe clips, but I did not put much stock in that as it appeared they did not use cyclists for the test. Others tested cyclists and triathletes and showed less muscle exertion at the same output with clipless pedals. As one naturally lifts the foot that is not on the downstroke when pedaling it makes sense that one would be more efficient with straps or clipless.
Spinning/form: Being strapped/clipped in makes spinning and "pedaling in circles" much easier in my experience. In turn that allows for better blood circulation to supply energy and carry away waste and less wasted energy in the form of physical deflection of the bike and side to side tracking.