Product review (4 stars): They work!
After reading lots of negative reviews on magnetic vent covers, I bought these BestAir Magnetic Vent Covers from a home improvement store by me. I was putting them on ceiling vents in my basement, so I was expecting them to fall off, and I would have to take them back. But, to my amazement, they did work! I even thought after air was blowing through the vents they would fall off, but they didn't. The best advice I can give is to cut them down so that they have maximum contact with the outside edge of the vent. They seem to need a lot of surface area contact in order for them to stay. But when they are cut correctly, they work perfectly. I rated them 4 stars instead of 5 because I have not owned them that long, and don't know how they perform over the long run.