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 Concept 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.

K&F Concept Adjustable ND filter with a very good price-performance ratio in top quality
You have to try a bit, since there is no scale, but with a little practice you can take great pictures.
(Picture was taken with a 2sec exposure at max ND filter value)

This is my first K&F Concept variable ND filter. Got it to use for my video needs. It’s a solidly built filter with reusable packaging for storage. For the price point, I love it for taking videos. Especially on bright days when focusing in, this helps give a bit of ambiance and subject focus to my videos. I can’t speak to photos as I’ve not had the chance to take this out for any landscape shots. If I do, I’ll be sure to update. For the price point I find the 10 stops to be a great addition to my on the go gear for video needs.

I hope this review or even just the pictures were useful in your decision to purchase or not to purchase.

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Really a very good ND filter!
I tested it directly in Hamburg and in the forest and it's just perfect!
The picture is very sharp and just looks great!
I put my two favorite pictures up with Rein.
The resistance to make the filter lighter or darker is very good and pleasant.

Overall a very great product!

I ordered this to use with my favorite walkabout lens, in an effort to get myself back in the habit of taking photowalks on a more regular basis. I had to stop because of my mobility issues, and it's taken me years to get back to a point where I might be able to manage a slow 20-minute walk on less-than-smooth ground, but it's time.

The filter size is accurate and it screws easily into my lens. The coatings are smooth, with no ripples, bubbles, or uneven appearance at any point in the rotation. The ring rotates easily and smoothly, and it's easy to see the glass visibly darken as it's rotated.

The photos come out crisp and clean, without the haze that's often attributable to unpolarized light, and unlike with some lower-quality ND filters, without any hint of a moiré pattern.

This is a third K&F filter that I own now, and it continues to impress me with the quality and value. This was an unknown brand for me and I can wholeheartedly recommend it to anyone now that I have used three different filters from them myself (UV, CPL, and this variable ND filters).

I am used to caring a set of ND filters with me in a case and I did want something more compact for different conditions. My approach is always to be minimalistic and to carry less. We travel a lot (a lit less now "thanks" to covid) and carrying less gear with me for convenience (and back saving) reasons is always nice. This filter performed superb on my recent trip to Turks and Caicos. We were in many super sunny situations and traveled a lot around the island. I am not a professional photographer but I would classify myself as an "advanced amature" and to my eye this filter is perfect. No color distortion that I can notice, no artifacts that I can see, great range, and convenience of multiple ND filters in one mounted on the camera! It rotates smoothly, stays in place, light, and seems to be very well built. Had it in some sandy conditions on the beach and it held up very well. Definitely recommend checking it out.

K&F Concept's range of filters have never let me down and neither has this variable ND3-ND1000 ND filter.

This filter maintains accurate color throughout the entire range unlike other filters I've used, which is a huge plus since it saves me time in post by not having to mess with color correction.

There is no dreaded "X", even when I intentionally went too far. While I'm not sure the stated range is entirely accurate, it is close enough and provides a ton of functionality, especially given the low price point compared to other filters that offer such a great density range.

I don't experience any vignetting with this filter, which isn't as slim as others I've used. But I rarely get vignetting when using the 18-140mm lens that I tested this filter with so my experience may not be applicable to different lenses.

Stops are clearly marked on the filter, and the adjustment is smooth with a little resistance that offers good control while shooting.

That said there is one thing you should know about this filter - YOUR LENS CAP WILL NOT FIT. I wish they included a lens cap with this filter or at least stated this clearly in the description, because you'll have to either print a new lens cap or buy one that will fit over this filter.

For the 67mm filter, you'll need a 72mm lens cap.

Overall this is an excellent value for a high quality filter. I doubt you'll find another variable ND filter with this range and at this quality for a lower price.

After I needed a new neutral density filter for my DSLR, I decided on this K&F Concept model and was not disappointed. I'm an amateur photographer. Pros may have different needs, but I wasn't disappointed.

Out of the box this filter is well made, threads on and adjusts smoothly. The optics are quality and I was pleased with the photos. I did experience some vignetting when pushed to the limits, but that might have been caused with me having the UV filter still attached and like I said that was at the very extreme end of the filter position. Very impressed with the filter and it's quality. The photos were F8, ISO 100 and 1/160,1/25 and 1.6 sec. Would buy again.

I have other K&F's and they all perform as advertised. This one is no different. Well made, good images, works as expected.

"Real" photographers, such as Rick Sammon can't say enough bad things about variable ND's. I have used them for years and never once was disappointed by them. The bad mouth guys are always complaining about the "X" issue. Yes, if you turn too far, you'll get an ugly X across your photo. Here's the secret which as served me well for a long, long time. You ready? "Don't turn it too far". I don't know why that is such a tough concept to master. Of course, the lazy manufacturers could help the situation by putting stops on the filters, but they seem to prefer the complaints to a simple, positive move. For my part, I help myself by gobbing on some white nail polish on the three points in question, so I can easily pick out the "too far" points. I can turn it down to focus/compose, then turn it up to where I want it and shoot. That beats the heck out of trying to shoot the image blindly, or repeatedly screwing and unscrewing the filter to get the shot arranged.

Really, a reasonable price, a little common sense, and you'll be good to go with this filter.

I hope you found this information useful and/or helpful.