Not great, but okay
I was anticipating function similar (not identical) to sidecar; another functional screen. That's more my fault for not reading carefullly enough, I suppose.
Anyway, it worked perfectly the first time I tried it, which is a nice surprise after reading the reviews. (For everyone having trouble I wonder if they granted permission to the app to record the screen; failing to do so would definitely make for "Doesn't work/nothing but a blank screen" results.
This doesn't give you another usable screen: Just a (slightly delayed) mirrored display inside a browser window. You could set the browser to full screen and have a non-functional, mirror display on your phone/tablet. If that's something that would be useful to you, this is a great free solution.