Product review (5 stars): They work great on ceiling vents.
Given all the poor reviews of these I was doubtful but figured I could always attach extra magnets or screw them onto the vents. However, they work fine. They need to be cut well to size and I had to rub all parts of them onto the vent a couple times using some pressure. Having done so, they stay in place and significantly reduce the cold air blowing out of ceiling vents.