Diffuser filters are used to blur light, for example, or to soften it.
The filter does that very well.
In terms of optical quality, I have not noticed any major difference to Neewer filters.
Of course there is room for improvement, but the professional filters are then quickly beyond 200 euros.

I used it on a Tamron 90mm f2.8 and a Nikon D500.
A bit notchy when unscrewing (Neewer can do that better now) but otherwise no reason to complain.
You have to experiment a little with the incidence of light. When the light source is directly in front of you, the effect can be quite extreme, as the soft glow "overshines" much of the immediate area.
With light coming in from the side / indirect light, you get a nice "glow". This can also be used in product photography, for example, to soften highlights a little (eg in cars to soften highlights).
And the filter does that very well.

The price is absolutely fine. If you want to use this effect permanently (Hollywood glow....) you should invest more. For the hobby / amateur photographer, absolutely sufficient (portraits also get that special "something" - try it out)