Great price

Keeps the bugs out of our vents and coming into the house for sure. But definitely should've measured the vents before buying them.

They work pretty well for this

I cut them down, layer them with a piece or two of cardstock, stick on the dies, put them in a sheet protector, then put them in my file cabinet.

I read several reviews before purchasing these and wasn't quite sure I'd be satisfied. The reviews were all over the board from 5 stars all the way down to one. Let me just say this, people (in general) must not be very bright.
These magnetic vent covers work very well when you think about how magnets work. They need full contact to stick. Most if not all air vents have a small lever that sticks out and allows you to open and close your vent.
If you desire to fully cover the entire metal frame, you WILL need to make a small cut out to allow this lever to pass through the cover and provide a flat sealed surface. If you do not do this, the magnet won't fully attach, and the weight will make the cover fall to the floor.
Sometimes the magnetic vent cover will be to large for your vent. Again, magnets work when they are in full contact. If you have excess it will cause too much weight, and make the cover fall to the floor.
Finally, some people claimed that when the air was blowing, the magnet came off and fell down. Remember I said that most if not all air vents came with a lever that closes your vent?. Close your vent before using these magnetic covers.
OK, so now that the troubleshooting section of my review is over, let me tell you what I think of these covers.
First of all, as with everything available through Prime, they arrived swiftly via USPS., meaning they were curved a bit from being jammed into my mailbox. This did not affect them. By the time I walked from my mailbox to my house and opened the package, they were flat again. Although, mine hadn't been in the box very long. If you left yours in the mailbox, and they were curled, or bent, that would affect their ability to fully connect magnetically. Flatten them with something heavy before using.
I had ordered mine a bit large, because I like to make refrigerator magnets. I trimmed them down to the size of my vents, (the air WAS blowing at the time) pressed it down to make sure I had good contact, and haven't thought much about them since. I live in a multilevel home, and in the summer, the upper levels are much warmer than the bottom. I needed to cover the bottom level vents to redirect the cooler air to the upper floors. My husband and I were out for a few hours, and by the time we got back, the temperature was completely regulated.
I plan to buy more this winter, because we have the reverse problem when heating, and have many more vents on the upper floor.
Very very satisfied.

I've noticed after placing a few cut pieces the pressure to some kicks the magnetic up and off. So magnet isn't super strong. But it still does what I need it to do at least