Product review (3 stars): Works fine, I assume same quality as OEM
Silly accessory really.  Our microwave originally circulated into the house, and the previous home owner didn't care.  We did, so we had to go find this darn thing because the original damper that would have come with the microwave was long gone.  Cant believe how much it is for a little damper!  Also as you might expect it doesnt hold back "all" the air, as you can easily see along the sides there is a bit of a gap.  But between this and the damper leaving the house its not bad.  The inside of the microwave is cool in the winter mornings, but nothing to get worried about.

Why 3 stars?
Expensive for what it is, and doesn't even have a snug fit to hold out air.  Install was simple (onto the microwave at least...total install of duct work in the wall was a huge pain) and did what I expected when I purchased it.