only tried a few times, but very impressed so far

It is a variable and polarizing ND filter.
The ND is 5 steps, there are 2 steps, 10 steps (ND1000, 1024 to be exact).
The polarizing filter eliminates light reflections on surfaces, cars, water, glass and also enhances the colors of the sky, leaves, asphalt, etc... (search the internet for more information)

Personally, I use it for video so as not to lose the depth of field by closing the diaphragm (f9 next, I can now go to f2.3 without burning the frames of excess light). I haven't removed it from my 35mm in a while, I'm thrilled.

The filter is made of metal and glass, presented in a hard plastic case for storage. The protective and anti-glare layers of a greenish color are appreciated and you can put the tip on it, which holds quite well without coloring.
The lever changes the density of the filter and the other ring moves the polarizing filter. You have to pay attention to it because you can move both carefree.

In the pictures you can see the filter attached to an adapter, (step-up 49mm to 67mm) this is because I have filters of 67 and 49, my advice is to buy large filters and then "step-up" adapters.

About the image quality. Excellent: There are no dominants, no chromatic aberration, no loss of definition, no appearance of the terrible X of gradual ND filters.
I have a very expensive polarizing filter and I hardly notice the difference to this one.

As a negative point, although I said earlier, be careful when handling one of the rings so as not to move the opposite.

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I use the neutral density filter for my nature photography and landscape photography. Especially when photographing rivers, lakes or waterfalls, this filter, due to its variable darkening of ND2 - ND32, offers an extension of the shutter speed which creates a motion blur which looks very nice in watercourses. In addition to the ND filter, there is a second filter function, a CPL polarization filter. This is very practical for the photography of water, lakes and streams because reflections can be compensated at the same time.

I chose the 77mm filter variant and use the filter on a 12-24mm wide-angle lens. The build quality is very good, even with maximum darkening of ND32, the unwanted X effect does not occur. The inexpensive filter shows good imaging properties in all areas and is ideally suited for ambitious and creative hobby photographers

Truly excellent build quality. It proves to be robust and has excellent materials. Super recommended, also because a CPL in combination with a variable ND filter is really affordable at this price.

This filter polarizing and neutral density pleasantly surprises me, it does not have excessive price and the quality of the photos is not bad, I have taken several photos with the different degrees that subtract the light and the result is good, if you like the silk effects in the water, this is an indispensable filter.
It also has the polarizer that eliminates reflections on crystalline surfaces

For the price a top product.

... with color photography, the polarizing filter unfortunately produces a color cast for me, so I had to send the filter back. Bought for practical reasons because of the 2-in-1 function, but now separate fitter for polar and grayscale function have to be found. I didn't use/test autofocus.

The filter is very suitable for reducing the light sensitivity of the camera and thus facilitating long-term shots (e.g. of roads). The cross shadows mentioned in some reviews I can not perceive in any of the settings of the filter. I use the filter with a 50mm lens on a Nikon D5100. Water and glass reflections can be easily removed by simply turning the filter.

only tried it a few times but very impressed so far

I was given this filter by K&F concept to review, but this is my honest review based on using it for a few shoots of both photography and videography. The filter comes in a really nice plastic protective case, also included is a cleaning cloth in a pouch. The cardboard box itself is sturdy with a magnetic opening. I asked for the 77mm filter to fit my biggest lens and got some step up rings so I can use this on all my lenses and by stepping up you wont get vignetting. The filter itself feels very good quality and well built, the exposure adjustment ring allows you to choose between five stops, it feels like a fluid video head and will not move on its own. I didn't witness any colour shift, or any photo or video issues using this filter, also there was no sign of the black X some filters show when the two optics cross at their maximum point. I would definitely recommend this variable ND filter, its at a very good price point and by using step up ring for around £10 you can have this one filter work on all your lenses, that is a lot cheaper than a few sets of ND filters when your lenses have different sized filter threads. Using this on video at either 25 or 30 fps using shutter speeds of 50/1 or 60/1 this filter easily cut down the light to stop my footage being over exposed. I have added a review on YouTube and video demo, you can find this by searching youtube for this product description.