My first impression was...for the money...it's not half bad.
But that impression slowly evolved...
Build is very nice...plenty of grip. But in attaining a nice grip, you get a fat frame.
I went with the 82mm filter to place on my Sony 135mm f/1.8...probably the best lens I have.
I could not put the lens hood on with the filter attached. I have a Polar Pro and I can have the hood attached. Deal breaker? No, but I like to have options whenever I can...so that's a ding.

It comes close to the Polar Pro in color cast...almost. There is some there but for the money...quite acceptable.

What is not acceptable (to me) is the loss of sharpness. It's definitely taking a hit...if you look.
My 135mm is razor sharp...to a fault. I shot this K&F against my Polar Pro for sharpness.

The Polar Pro looked just as sharp as shooting without any ND filter on...and sort of what you pay for.
The K&F reduced the sharpness...in focus mag...I was never happy with what I saw...you think it's almost there...and you go past it (the sweet spot). You go past it because it is NOT there.

I would pass on this filter. If you need something razor sharp in the image with blur around it (typical usage of an ND).

The color cast part was very good...almost gone.

But the overall softness in the filter...is a no go...for me.