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.

Does the job: protects the lens of the lens. Matte appearance and a bit rough feel, but it's just an aesthetic detail.

I've seen various films that noted the use of diffusers. I really compared a lot of black matte filters. This one is perfect. I get the softening without noticing it. This is really a high quality filter. I am very happy with it.

This is a great product. Easy installation. I love the elegant matte black. This will definitely keep my kids out of my room

The camera stand is made of sturdy material and has a high quality of matte surface!! The most impressive me it is the camera stand has multiple adjustable tight handles that allow me taking product pictures from a wide angle.

I bought it for its minimalism. All matte black and I love it. With my Nikon Ai F lenses it suits luxury

I really like the ND filters, they come nicely packaged in a handy little box that even fits in the pannier easily.

Now to the filters. After some tests I can only warmly recommend them. They are very sturdy and look very high quality! Also, they fit perfectly and you don't have any fringes or anything in the image (which is the case with some comparable filters).

There is not much to say about the function. They're just filters and they do what they're supposed to do. If you use these filters correctly, you will certainly be satisfied with the result.

Clear purchase recommendation from me!

Finally found an ND filter which can be installed inside the Smallrig lightweight matte box 2660.
I did have to remove the holder frame to make it fit. This can be done by carefully pulling appart the frame sides with the lips. In doing so you can op out the filter while keeping the frame intact.
Filter quality seems to be ok and it comes with a very well designed holder with an extraction ribbon which avoids having to touch the surface of the filter when taking it out.