Product review (2 stars): Magnetic Strength Poor For Wall Vent in Newly Constructed Apartment Building
While the item sticked to the vent if there was any forceful air flow (like opening the apartments main door or closing the bedroom door) the vent cover would pop off and fall to the floor.

After looking at the other options and reading the negative reviews on Amazon for similar items,  I settled on this brand.  I used it to stop air flow from a wall vent in a new LEEDs (Green) certified apartment building on Dolores Street in San Francisco.  The architect seemed to think residents needed free air flow from the outside to the inside of the apartments (all the time) and there is no way to stop it or close the vent.  When the temperature fell to 34 degrees this December, (a record low for SF) and the heaters couldn't keep up it was time to seal these vents up.  If you supplement these with clear packaging tape or velcro they will do the job.  Too bad the magnetic power was not enough to do so alone.