Delivered in a plastic box that protects it well, the filter looks very nice and the polarization works well. I will also use it mounted on the lens to protect it. Useful and high quality

I got this to see how it would work and to add it to a project that is still in progress. I haven't mounted it yet, but I took it out of the box and tried it out to check it. It seems to be able to work just fine and will be a nice addition. I don't expect any problems, but will update if that changes after actual mount and use.

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

The filter is mounted directly on the lens. You can always take it, it is not heavy and you will get great effects with it!
Practice & optical quality:

many voices will say, such inexpensive filters simply can not reach the quality of a brand filter and also I was until now someone who would have immediately affirmed this, so much I was convinced of my B&W, Rodenstock, Haida and Co. filters. But now the surprise, the K&F filters show no abnormalities. No unnatural color shifts, no clear loss of sharpness, hardly any flare and ghosting susceptibility, simply nothing where I would say at first glance that it would be a reason for exclusion. And I think this fact has opened my eyes. Either K&F has found a manufacturer here who also does this for the brand companies, or as I think, they have simply inserted a second layer in the factory and instead of the brand name, simply lasered K&F on the filters.

Mounted on a wide angle lens, the rendering on water / lake pictures is perfect.
Rigid carrying case to avoid damage.
Good image quality. I did not notice any vignetting or color modification.

Mounted on a 7DMKII, I haven't used much yet (I'll amend my message if necessary). The piqué is honestly good, even at full aperture (in this case, except at the edges, well-known chorus), and is well worth that of a "classic" branded lens (I'm not talking about extraordinary and overpriced myths). The large aperture allows for a beautiful bokeh. A sun visor mounting groove is missing, the front lens barrel has some play - probably necessary for autofocus - and the latter is a bit noisy (without excess). Note that the autofocus in itself is pretty correct without being a war flash (in photo, for video, that's another question). In short, to avoid in autofocus for video, but for stills... it's a real pleasure to find a solid and light 35mm, sub 2.8... at a price that only squeaks snobinards teeth !

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.

Flawless and effective product.
I finally mounted a Helios 44M lens (look for it and buy it too - results from prime lens) which, with another K&F adapter (EOS / M42), I can also use on GX80.
Punctual and reliable.
Very satisfied.