***Update***

I have now also bought the 72mm version in the flash offer and am just as happy with it. I now use the variable ND filters as a "quick" filter when I'm on the go. If I want to expose more professionally, I use my K&F square filters. These are also highly recommended, by the way!
The 72mm round filter has similar good properties to its big 77mm brother. It has slight color casts in the maximum range, but impresses with its consistently good image quality and workmanship.

________________________________________

The filter is very high quality and comes with an extra cover. The filter is made of metal and glass. The two rings can be rotated very smoothly against each other.

The variable ND function works very well for me. Both discs are perfectly matched to each other. I could not see any loss in image quality in my test images. The design automatically results in vignetting and sometimes a color cast. But nothing that can't be quickly worked out with image editing software!

A great tool for filmmakers and photographers to be able to do a variety of creative work with just one filter.
The price of around €50 is basically justified, but the price here could be closer to €45.

All in all, a recommended purchase.