Personally, I don't use them. My very first hive came with 2 pre-installed... and fully propolised! Tried them for a couple of years and then discovered the non-moving-parts alternatives, such as Canadian clearers, and have not looked back.
I'm a big fan. Their no good for osr honey ( too slow to clear )but excellent for clearing summer honey as there's no need to go back in any specified time ( e.g. distant out apiaries) assuming all your supers and roofs are bee tight