Product review (2 stars): Don't work as advertised BUT...
All the prior reviews are acccurate and these don't work as advertised for the purposes of covering wall vents. However as weak as these magnetic covers are they can potentially work for you. Here's the solution I found that works ok:

1) First make sure to cut them exactly to size...the smaller the better as this allows them to hold their own weight much better.
2) Double them up. After you cut it to size use 2 covers back-to-back on wall vents. They will hold this way.

If all else fails you can always use those mini-magnets which are obscure and powerful enough to hold these up and get the job done.