I have two of these variable ND filters with 55mm and 67mm filter threads. The filters make an absolutely high-quality impression, which is also supported by the packaging, in particular the storage box.
The filters have a scale on the aperture ring that starts at ND2 and ends at ND400. There are 8 stops in between. Unfortunately, the respective extension factor for each stop is not documented, so that the exposure time is difficult to calculate. Here, you have to get the best result by trying things out.

Review of "K&F Concept 67mm Magnetic Variable ND2-32 Filter (1-5 Stop) Set for Phone Lens Camera Mount Comaptible with iPhone 15 14 13 12 11 Series, Magnetic Neutral Density with Filter Holder" currently at £49.99:

I used this with my Samsung S20FE phone and it fit well even with case on. The filter is 67mm and fits over all the cameras on my phone easily. It has a nice smooth action for adjusting the ND level. It has a magnetic clip for the lens which is nice, but I would have liked a standard screw mount so other filters could be used.

It is quite expensive for what it is, especially as I can't attach other standard 67mm filters to it, but it does its job well so I can't really mark it down.

I recently got the K&F Concept 67mm Black Diffusion Mist 1 4 Effect & Variable ND2-400 (1-9 Stops) ND Lens Filter and it is incredible! This filter has adjustable levels of density, allowing me to get the exact effect I'm looking for. Not only that, but it's multi-coated with 28 layers of Nano-X Series and made with high-grade materials that feel sturdy.

I am very pleased with this 67mm ND filter from K & F Concept. I am using this on a Sony 24mm 1.4 G Master lens (Bonus it will also fit on my 85mm lens). This is great for time lapse photography during the bright afternoons in Florida. Examples of the scenes I've shot are, the summer rains coming through over the water, or of the city scenes at the urban farmers markets, etc., this filter has exceeded my expectations.
I purchased at medium sized backpack from K & F 2 years ago that has been a hearty addition. The backpack has protected my gear on trips to Thailand, Hong Kong, Philippines, and Colombia. So, I figured why not try their other products. I'm looking forward to using these filters for many years to come.
Great buy at a great price.

Over the past year or so, I’ve tried maybe a half dozen K&F filters uniformly finding them to be among the best I’ve ever experienced. This high density ND 1000 has met my expectations of excellence that this brand has now established with me.

There’s little to say other than it’s thinner than other filters of any sort meaning it has less of a chance of creating unwanted refractions once mounted. Like all filters, it must be used alone. If added to an existing UV or clear filter many used as a lens protector, the two can and likely will work to degrade image quality. This refraction issue is a good reason not to mount the UV or clear filters as a lens protector. A lens shade works almost as well as a protector but won’t interfere with a filter.

In use, this ND 1000 does filter to 10 stops (eyeball estimate) without affecting the color or introducing other unwanted effects. Like so many things photographic, the good stuff costs but also performs.

1000 yard version

This is my first variable ND filter and Im very happy with it. I got the 67mm 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.

Apart from the UV filter which I don't use, I find that the polarizing filter is excellent, and the ND 1000 filter as well, but the brilliant thing is their magnetic mounting, it works smoothly and quickly, for me an excellent product.