Product review (5 stars): Does a great job of keeping the outside out.
I didn't buy this for its cold air efficiency, although, I think it has made a major improvement.  I bought this because my neighbor began heating his house with wood, and if you didn't get the clothes out of the dryer the instant they were done, they would begin smelling like smoke due to the proximity and typical wind direction.  This vent close solved that problem for me in its entirety.  I was shocked how well it worked.