ND filters are among the essential accessories in a photographer's bag because they allow you to take shots that would otherwise be impossible. Since most cameras do not allow you to go below 100 ISO, in good lighting conditions it is impossible to have enough shutter speeds to take photos that also capture movement.
In this case it is a variable filter, which can become more or less dark simply by rotating the front ring, continuously and not in steps. It therefore goes from a single reduction stop up to 5. This means that you can go from 1/80 of a second without a filter to 1/5, which is not a small amount at all.
The overall quality loses a bit in terms of sharpness and detail, but from my point of view not so much as to become annoying. Of course, I tried it with a Canon 6D which has a 20 megapixel sensor, with 24-105 F4 L, on cameras with higher resolution the result could be worse. Obviously the result is inferior to what you would get from a non-variable filter but the flexibility somehow comes at a price.
Even at 24 millimeters I did not notice vignetting which could however appear on shorter focal lengths given that the filter is not very thin.
The selling price seems more than adequate to me given that it allows you to save on the purchase of filters with different opacities.