These are very useful, and I started last year with 2 of these for each hive. I probably only needed 1 per hive plus an extra or two, but you can't really beat the price either compared to what you'd pay elsewhere for the same quality propolis trap.
I use one of these propolis traps on the tops of my hives even when I am not using feeders. I always like to keep an extra deep super on top of my hives that I stack frames in during inspections, so the propolis trap keeps the bees out of the extra super and also gives me an easy way to puff in a little smoke before releasing the bees.
This propolis trap is thick enough that bees don't fight each other through it, and I have used it for combining hives rather than newspaper. Newspaper is hard to work with if there is any wind and that makes a bit of a mess after the combine is done. The propolis trap will hold it's position even in wind while you are grabbing the other box and it comes out of the hive in one piece when you are finished with combining the colonies.
I use the frame spaces below the propolis trap to hold it off of the tops of the frames.
The only real negative that I can say for the trap is that it is too thick to use the upper latch hooks and when using the lower latch hooks the latch doesn't really snap into place as when fitting in a queen excluder or a feeder between boxes. I haven't had a problem with the latches coming undone, but if someone worries about bears, they might want to shim it to fit tight.