We used the UL approved foam inserts and the child proof socket plugs in two different houses for two different situations.
House #1: 1950s brick and block bungalow, with brick, block, plaster and mud walls (outside to inside) and no wall insulation. Due to the brick and block construction, it would be difficult (we are told) to have insulation blown into the walls. The house had typical double pane vinyl replacement windows when we bought it, as well as R19 batting insulation in between the attic's floor joists over the original rock wool insulation.
House #2: 2007 stick built with 6" exterior walls, good vinyl double pane windows, R49 blown insulation in the attic.
House #1: This is our Empty Nest- a 1410 sq. ft. bungalow that we've renovated mostly via DIY (with some work contracted out where that was the best/safest/sanest option) and which we heat mostly with a pellet stove. We've added a radiant barrier to the attic, as well as R30 batting rolled out perpendicular to the floor joists and the existing insulation. (The R19 between the joists was in good shape so we left it in place.) The pellet stove, which supposedly can carry 2000 sq. ft. still required help from the gas furnace on the coldest days. Early this fall we went all honey badger on what air leaks we could address without tearing things up: sealing penetrations from the ceiling to the attic (light fixtures, ceiling fans, exhaust fans, etc.) and adding the foam inserts behind the switch and outlet covers as well as the child proof socket plugs in outlets that are not used continuously. We also caulked around the quarter round trim on the base boards with paintable, clear drying caulk. The hardwood floors have been refinished a couple of times and sanding left a gap. We could feel the draft coming into the room from this gap.
Since we did all of these things at once it's hard to say what, exactly, the foam inserts contributed- but I can tell you that there is one wall of our house that takes the weather and wind coming from the west more than the other walls. Before adding the foam inserts and the outlet plugs to the outlet on that wall, I could feel the cold draft coming into the room through that outlet. Now, not so much. It was sort of difficult to tell, sometimes, if I was feeling a cold draft or simply conductive/convective cold from the wall. Believe me, when it's in the teens or below and the wind is hitting that wall, the interior plaster wall is COLD. But yes, the inserts did help us, as did the socket plugs. Now when I put my hand next to the outlet on that wall, I can still feel the conductive cold rays emanating from the wall- but at least the air is not blowing past my hand.
House #2: This is the home to which we hope to retire in a few years. One side of the house faces an open field with no wind break. On the other side of that field is two+ miles of unobstructed water. There are times in the winter when the wind comes down the river and across that field at over 60 mph, and that's without any storm system in play. That's just because it's Tuesday and it's a great day for wind! Even though there are 6" exterior walls and good windows, and the house was built by a local builder who has lived in this climate all his life and he knows how to build for it, and the house was built for a retired builder who had common sense things in mind when he built it like the appropriate materials and construction for the climate (and we ended up buying it, long story but our good fortune) that wind is wicked. We could stand in the rooms on the back of the house and feel the wind coming in, as others have said, through the prong openings in the sockets. I don't mean a cold draft coming in through the prong openings. I mean you could feel the wind coming in through the prong openings. If it had whistled as it came through the prong openings I would not have been surprised.
Did the foam inserts and the socket plugs help? You betcha they helped.
This is a small thing to do- it's easy and relatively inexpensive- so why not? It did help us.