I received the adapter last week and luckily was able to have the long Thanksgiving weekend to play around with it. I'm using it on my A7IV with the Leica 40mm f2 Summicron-C, a tiny little lens, even with the adapter it's smaller than my other small lens, the Sigma 45i. The adapter is a really neat piece of kit, though quite pricey at $400, which could be spent on another lens itself.
The AF behavior with the Leica 40/2 is as Bastian described in his review--only accurate within the central part of the image. It uses Eye-AF within that central zone, but it drops it outside of that, which I found to be a bit problematic for composing many shots.
I would like to use this with the Voigtlander 40/1.2 or 50/1.2, but I haven't seen whether these lenses only AF well within the central portion of the image. Has anyone tried these two?