I have two of these variable ND filters with 55mm and 67mm filter threads. The filters make an absolutely high-quality impression, which is also supported by the packaging, in particular the storage box.
The filters have a scale on the aperture ring that starts at ND2 and ends at ND400. There are 8 stops in between. Unfortunately, the respective extension factor for each stop is not documented, so that the exposure time is difficult to calculate. Here, you have to get the best result by trying things out.

For my Nikon z50 with its kit lens 16-55mm along with 35mm and 50mm primer lens. Bag is very stylish and sleek. Wish it was a bit narrower. But overall a keeper

55mm Circular Polarizers Filter, K&F Concept 55MM Circular Polarizer
I've only used this pola a little bit. It is well made and the knurled ring is easy to smoothly adjust.
Only two nit-pics -- There is not mark on the filter to indicate the maximum effect, and the very sturdy rings interfered with the filter mount on the lens. This is not enough to make me regret buying this pola filter.

For me this filter serves its purpose and it is currently the best filter at this price

As a full time filmmaker I own several K&F filters. They are cheap but have a solid performance. Especially fir your output medium is Social Media or even TV. (I would not use them for Film related projects).
One of the problems this ND filter solves is especially when you using a Matte box and you have the ND attached to it. It is really hard to "feel" the filter without looking at it.
I have smudged the lens with my sticky fingers several times because I can't really feel the ring while holding the camera.
This filter has to nice handles that you can tactically feel easily when using a matte box.
Great addition for my camera bag.

A good quality "square" filter system for photography. One thing to note if you are considering this system, the adapter ring kit does NOT come with a 55mm ring at this point. I raised the issue with K&F through their Facebook page and a response to that said the point would be shared with the appropriate people. Who knows, maybe it will come with a 55mm ring soon. If you need such an adapter for one of your lenses, you have to buy it separately. That means it's well made and sturdy enough to last for years I think and apart from 55mm there's a fair choice of adapter rings in the kit, eh

It takes a little getting used to, but the value of a high quality mount, ND filter and a full circular polarizer with several adapter rings makes it a bargain. I can even use it on my 14-30 NIKon Z Len 14mm 14-30 with no vignetting once properly attached. An awesome kit. The ND is excellent and neutral. Recommended.

This K&F Concept 55mm Variable ND filter fits my lens well. It is a well made quality product. The filter worked as well as some of my more expensive filters - very happy with it.