Product review (5 stars): These do work for ceiling registers 6-16-03
I looked around for magnetic vent covers for my basement wall and ceiling register and could not find any with real positive reviews so I took a chance on these being they are made from BestAir.  That being said it has been a few weeks now and I have not had any issues with them falling off.  I think the key is what kind of register you have in the first place.  Mine have a wide frame so there is more area for the magnet to stick to.  Also cut them to size and I even cut holes where the screws are because they pop out a bit on mine.  Im glad I took a chance on these because they do work!

Update:  Still holding strong 6-25-2013 and I bought another pack to use in an entry room for the floor registers and they work great as well. The temp in the room really changed from just having the vents shut to completely blocking the air with these.  A+ energy saving product!