I already have quite a few filters at home, including the K&F Concept Nano-X ND Filter 77mm Variable Neutral Density Filter ND2-32 for €75, which I use on my wide-angle lens, for example, in bright sunshine (for wide-aperture shots) and for long exposures, and it's really good.

Now I had the chance to test the variable ND filter from K&F's PRO series, a filter for a whopping €280. Of course, if you're going to work with filters, you should use high-quality glass on a high-quality lens, but a filter for just under €280 isn't cheap, and this investment should be carefully considered. What I can definitely say is that the filter is at the top not only in terms of price, but also in terms of performance and workmanship.

It starts with the presentation (packaging) of this filter: a neatly crafted case with a magnetic lid, suede appliqués, a soft, lint-free interior lining, and a side-mounted strap that allows the filter to be pulled upwards.
The filter itself is extremely well-made, and the two brass tabs make it easy to operate.
It offers a very pleasant resistance when adjusting, not too heavy, not too light.
The glass shows no defects, and the threads are also cut absolutely cleanly.
Regarding the image quality/performance, I can only say: TOP!
There are no color aberrations/color casts, or anything else negative. This means I can work with my lens at full aperture in bright sunshine, and if I like, I can also use the filter to create long exposures with a slightly closed aperture.
As I said, the image quality is as good as if I hadn't had a filter on the lens at all, and that's how it should be.
The images I uploaded (undemanding test shots) were taken at full aperture and against the sun.

My conclusion about this filter:
Very high-quality workmanship/flawless image quality/excellent feel. Very high price.
Everyone has to decide for themselves whether such an investment is worth considering.