K&F now produces very good and affordable accessories and ND filters, and it is my first choice for every new lens.

Very good for its money, unfortunately on its "dark" side it makes this X pattern that other comments also mention, so it's not great for holding light. For simple video in the sun, and a few long exposure photos in the morning or afternoon, it's very good! My only complaint is that it doesn't stop when it reaches the max, and continues freely.

I bought it and used it on a Canon 70-200...from 135mm and onwards, everything is soft, and I thought it was my eyes or the lens's fault. The quality of the glass is terrible, so I threw it away because I couldn't sell it. I will go for Hoya, they will take me hunting

Whatever is the most value for money as a variable filter. All cheap filters at their last stop create this "X" pattern, that's why the expensive ones are sold with ND Stops 2-5, 6-9. It doesn't give bad results, as long as you work with the stops that don't create distortion. I used it a few times in scenarios with strong light and I am quite satisfied.
One negative thing about it is that it doesn't stop when you reach the last stop, which means that you either have to look where it is on the lens or empirically bring it to the desired exposure.
If you want a cheap filter to do your job, buy it blindly. If you want top quality, you will have to spend a lot more.
This specific filter was used in the following video:
https://youtu.be/4A_kXBZ90eg

Very good, but it requires attention when screwing/unscrewing, not so much during its initial installation, but after use, because if you tighten it a little too much by mistake while adjusting the brightness, it becomes very difficult to unscrew. It is

Nice and good BUT when you accelerate at a speed of 13-15 seconds and above and you close it, it starts to vibrate, only this, everything else is fine

Towards the end of the route, when you go for the most secluded (darker) spot, I notice that the darkening is not uniform. It appears in solid colors on the background (blue sky). It's not tragic to say "oh no," but in order to be accurate, I have to point it out. Otherwise, it is definitely value for money and has a nice gradient.