Worked ok for me.
I noticed it effectively creates a copy of all your artwork in AppData\mb_RoundedArtwork\temp for all libraries, not just the one that is currently active.
I imagine that might take up a lot of space if you have a large library.
It also created 9 copies in this folder for the cover for Nirvana Nevermind 30th anniversary, which is a bit weird because there are 13 tracks on the album. It's listed as a single album in Musicbee.
It's grabbing the album covers from AppData\InternalCache which has the 9 nirvana covers in there so the plugin didn't cause that problem, it's probably my terrible test library.....
The InternalCache folder is only there on the portable version of Musicbee. There is no InternalCache folder in C:\Users\<Username>\AppData\Roaming\MusicBee so it's probably not working for people running the installed version.
edit: I've only had 4 hours sleep in the last 2 days so I'm probably missing something but I can't see anything in the source code that tells Musicbee to display the rounded temp images instead of the original artwork. How are you doing that? Nevermind, I see it.
edit2: If it helps. the internalcache folder for the installed version is located here on my machine
C:\Users\<username>\AppData\Local\MusicBee\InternalCache