I already have a few filters at home, including the K&F Concept Nano-X ND Filter 77mm Variable Grey Filter ND2-32 for 75 euros, which I use on my wide angle, e.g. in bright sunshine (for open glare shots) and for long exposures, and it's really good.

Now I was able to test the variable ND filter from K&F's PRO series, a filter for just under 280 euros. Of course, if you are already working with filters, you should also use high-quality glass on a high-quality lens, but a filter for just under 280 euros is not a cardboard stick and this investment should be well considered. What I can definitely say is that the filter is at the top not only in terms of price level but also in terms of performance and workmanship.

It starts with the presentation (packaging) of this filter, cleanly finished bag with magnetic lid, suede appliqués, soft and lint-free lined interior, a sideways strap with which the filter is moved upwards by pull.
The filter itself is extremely well made and can also be operated with the two brass noses.
It has a very pleasant resistance when adjusted, not too heavy and not too light.
The glass shows no flaws and the threads are also cut absolutely cleanly.
As for image quality/performance, I can only say: TOP!
There are no color errors/color tint or anything else negative here. I can therefore also work with my lens with an open aperture in bright sunshine and, if I like, take long exposures with the help of the filter with a slightly closed aperture.
As I said, image quality is as good as if I had no filter on my lens at all and that's the way it must be.
The pictures I uploaded (undemanding test shots) were taken with an open aperture and against the sun.

My conclusion about this filter:
Very high-quality processing/error-free picture/excellent feel. Price very high
It is now up to everyone to decide for themselves whether such an investment is an option.