I have a small lens that is prone to flaring without a hood, so the usual step down rings with my filter(s) was a no go.
I like K&F and have used their products over the years with no problems. I ordered the 49mm to solve my flare problem.
It comes in a unusual circular plastic case, not the typical clamshell filter cases or the cheaper clear circular cases. The really interesting part is on the inside. Someone had the wherewithal to install two rubber(silicon?) grip pads...so no matter what size filter is inside...it won't move around within the case.
I'm not sure the case itself is going to hold up though...I had a semi difficult time trying to get the case to close securely...you have to...well...I had to mess with it a bit. Something they can improve on.

Cases...meh...how is the filter. It's very good. It's low profile...my lens hood doesn't even know it is there. The filer has great knurling around 75% of the filter, and has enough thickness that you won't have any trouble finding it without looking...if you need to adjust (if you have a hood with a finger slot that is).
The CPL turns smoothly and easily with zero slop. It won't move unless you want to.

I have included two images of a reflection in one of my bird baths...as you can see...it does a great job at cutting reflections. I will say it is not the strongest CPL I have...but that's not a bad thing either. I have some that are like rocket fuel for a go cart...simply too much. This CPL is balanced...and I did not notice too much light drop off at all when adjusting polarization. I had to use the reflection to find the strongest position for the filter.
It's a well balanced CPL...not too much...and just enough. It gives good saturation without knocking your colors about.

It's worth the coin...thumbs up!