Product review (4 stars): Great Solution to keeping unwanted air from coming out!
We use these covers in the summer to push more air through our vents to our upstairs. We did cut them to fit the size of the vents we wanted to cover and haven't had any problems. Some reviews stated they fall off easily - the only time ours have fallen off our vents are when they are pulled off (we have young children) or knocked off.
Our half bath on the lower level of our house tends to get very, very cold while using the A/C during the summer (even with closing the vent). These have been absolutely wonderful to block the air that still pushed through.
After seeing the difference in our house, my father bought a few packs for his house. While the upstairs may be comfortable for them, the downstairs was always too cold. Some of their vents are even on the ceiling. He, too, has had great results. It's not as cold, and even using them on the vents on the ceiling, he has not had any fall off (he did cut them down to the size he needed to cover the vent as well).
This price is a great price and I'd suggest these to anyone that would like to keep cold or warm air from coming out of certain vents in their house!