I would say that at least 50% of my service calls for no power to lights/outlets are a direct result of push-in connections. First question I ask when I walk through the door is "where did you plug in the space heater"? I've seen many many melted plugs like the one posted earlier. They are always back- wired.
With a little practice and a screwgun, you can use the screw terminals and keep up with the backstabbers!
Pat
