I use the 52mm version on a Sigma C 35mm/1.4 for Sony and can therefore only say something about it.

In brief:
(+) good sharpness and color fastness
(+) good workmanship and feel
(+) improved adjustment with handle lever (no problem even in cold weather with gloves)
(+) infinitely variable and versatile
(+) nice and practical leather case for storage

First I asked myself what the sharpness and color fastness look like with the variable ND filter. I've had such bad experiences with other products here in the past that I ended up only using these filters in an extreme emergency. You can achieve such nice results with ND filters. Wouldn't it be nice not to have to carry around 5 different filters and still get good results?

In addition to being used for filming, the filter is particularly interesting for long exposures (e.g. in water) and open-glare shots in sunlight.
The filter is easy to use in all three cases. You just have to be aware that you can no longer use the lens hood due to the small handle lever.
This lever is so easy to use that you can also use and change the ND filter while filming. You won't notice any shaking on the tripod.

The SHARPNESS is surprisingly good and I was also unable to determine color changes/color casts/etc. That's why there's a big plus here.

The ND value can be adjusted continuously. However, this also means that the actual value is not discernible. There is a scale with small squares, which are in the right place even when the filter is screwed on - but this does not replace precise information. So if you would like to let the amateur mathematician out and calculate exactly how you can/must adjust shutter speed and aperture, it will be difficult here.
For me personally, however, this doesn't matter in practice. I then simply take a few minutes more time and, if in doubt, I will get the desired result by trying it out.
Especially when filming, however, I can already see what the result is in live view and here the filter is only used for me to adjust the final exposure. It also serves this purpose very well.

The workmanship is of high quality. The filter is not the lightest or thinnest, but it looks very robust, is light but not too easy and, above all, the thread can be screwed on without the snagging and tilting that is otherwise common with filters. The screw-on handle element underlines the high-quality condition of the filter and supports the already pleasant feel in a special way.

I also find the stylish leather case successful. It is very well designed and the filter can be removed from the bag easily and without fiddling with a simple mechanism. All you have to do is pull on a band on the side, which runs through the bag and thus pushes the filter upwards.

Conclusion:
Good filter that gives surprisingly good results and is very pleasant to use.
The workmanship and quality result in a very good price-performance ratio.