The filter works best if you have a lens that is greater than 35mm. When the lens is too wide angle is when you get the X looking area

I'll start by saying that, for what it is, this product is pretty good; be aware this class of device (variable ND filters) have built-in limitations.
First the good parts: the product feels solid, the knurled ring is useful for getting it on and off, and its minimum and maximum filter settings are clearly marked. I will take K&F at their word the spill-resistant coating works; it definitely does not seem to accumulate fingerprints.
Then the built-in issues: the big one being the blotchy, uneven filtration when you close it down too far (the dreaded "X" effect). On my camera (Sony A7iv with 24-105mm lens) this occurs at roughly six stops of filtering. Your mileage can and will vary, but expect to see the X at some point, especially at the wide end of a zoom. Another problem is the "roughly" part -- the markings on the filter do not necessarily correlate, at all, with F-stops as metered by the camera. They are at best approximate. A third problem is the Min/Max zones are marked but there is no tactile feedback or detent to tell you when you exceed them.
More expensive ND filters may not have these problems but they definitely come with a pricetag. If this filter works OK on your camera and lens and you can live with the limitations, go for it.

This is my first variable ND filter and Im very happy with it. I got the 77mm version because its a very common size and I already have three lenses that it could fit. Plus with adapters, I can get my smaller lenses to work as well.
The filter. Overall its great. Metal construction with a very smooth ring.
The stops are marked on the ring and the overall construction is very good.
Again this is my first variable ND and so far I'm enjoying it very much.
Price is affordable too.

ATTENTION! This filter is not compatible with wide-angle lenses.
On my Sigma 16mm it produces a blue X when set to ND1000.
The manufacturer was very nice and offered me a replacement filter.

WARNING! This filter is not compatible with wide-angle lenses.
On my Sigma 16mm it creates this blue X when set to ND1000.
The manufacturer was very nice and offered me a replacement filter.

Challenge number 1) The K&F Concept 77 mm Variable ND filters I purchased arrived later than promised and I was not able to use it for my 9 day photoshoot. Challenge number 2) It fits on my camera lens but needs time and patience to attach to the lens but it does fit. Challenge number 3) No positive stops, no detents to let you know how far you've gone (my ND filter 1.5-10 stops). In addition, if you are invested in using your lens hood while shooting, it won't fit with these ND filters.

I'll be using the ND filters on my next photoshoot and will submit an update to my review.

As others have stated, when used correctly this filter does not produce the dreaded X. Nor does it have any color cast that I can see. The quality of construction is very good, and the ring rotates smoothly without being too tight or too loose. Quality product. Glad to have it.

ATTENTION! This filter is not compatible with wide-angle lenses.
On my Sigma 16mm it produces a blue X when set to ND1000.
The manufacturer was very nice and offered me a replacement filter.

WARNING! This filter is not compatible with wide-angle lenses.
On my Sigma 16 mm it produces this blue X when set to ND1000.
The manufacturer was very kind and offered me a replacement filter.

So I am a big fan of most K&F Filters and this filter is no exception. It worked great on my recent trip to San Deigo for vacation. The only reason I would not give this a 5-Stars is found when the filter is attached to my lens the lens cap keeps falling off.

Decent version, easy to rotate without play.
However, the sun visor can no longer be put on because the rotating ring has a larger diameter than the filter holder.

Thank you for the timely shipping, for the excellent packaging of the goods and the excellent quality of the goods.