Admittedly, I was a bit skeptical that an adjustable ND filter could really be good. You usually pay more for an ND filter in one strength.
I don't use a filter that often, but it comes in handy every now and then, for example when I want to take pictures with an open aperture of 1.4 in the sun or for photos by the water.
So I ordered the adjustable ND2-ND400 from K&F Konzept. I already have backpacks and a tripod from this manufacturer and I am very satisfied with them.

*** Delivery ***

As always lightning fast with Amazon, the filter came securely packaged with Deutsche Post.

*** Quality / First Impression ***

The filter is packed in a small plastic box, which can also be used for storage. Here I would like a recess inside so that the filter does not always slide back and forth.

First and foremost I bought the filter for my EF-M 11-22 IS STM lens (55mm filter diameter). I have a step down adapter for my EF-M 32mm 1.4 so that I can also use the filter on the smaller diameter lens.

The filter makes a high-quality impression. Nothing rattles or rattles. It attaches easily and turns buttery smooth. Not too loose and not too tight. There is a scale from MAX to MIN, so you can continuously adjust the filter to the desired strength between ND2 and ND400. However, you should not go beyond the MIN and MAX markings, otherwise there will be a black area in the picture.

The image quality is great, I can't see any loss of quality or loss of sharpness.

*** Conclusion ***

If you don't want to carry around several filters and change them all the time, an adjustable ND filter makes perfect sense. Also and especially if, like me, you don't want to spend so much money on several strengths at once because you don't use the filters all the time.
A landscape photographer who uses filters for almost all photos will certainly get an expensive plug-in system, which makes sense. But for the amateur photographer who would like to photograph a stream with soft water from time to time or with a wide open aperture in strong sunshine, a variable filter makes perfect sense. The price / performance ratio is really excellent with this filter. From me there is a clear purchase recommendation!