I have one on order my self I ordered it to allow me to use my digital Amiga joysticks to play Amiga, MAME and spectrum games and write ADF images back to disk.
Support for reading and writing Amiga floppy under WinUAE would also be nice, but at the moment just being able to write ADF images back to floppy to use on my Amiga is what I was looking for when I ordered it.
As I understand things their were hardware limitations with the mk3 version made real time winUAE write support unworkable which are fixed with the mk4 and that’s why the driver support under windows is so limited for the older versions of the card.
So hopefully in time we will see full support for the new version under WinUAE as I have never own or used a C64 at this time I am not interested in the SID support, but as I am interested in old games and Emulators in general this may change in the future if SID support is easy to setup.