In short, it works very well and is a good price (currently $86). Highly recommended. It is very versatile and arguably can replace several filters in your bag.

I believe they basically layer two CPL filter over each other. By doing this, you effectively get a CPL and a variable ND4-ND64 (2-6 stops) filter in one.

The operation is smooth, solid, and natural. The CPL is operated as usual by twisting the filter. The ND effect is separately operated using the orange tabs. Very easy to adjust both on the fly. The ND filter has stop markings, which helps with precision. Be sure the filter is securely attached to the lens because the filter operation is quite firm and adjusting the CPL/ND might loosen the filter from the lens if it is not secure. I appreciate the "gear teeth" grip on the filter itself for attaching to a lens. It makes it really easy grip the lens to both get it snug on the lens and also remove from the lens.

The build quality feels very good and solid. It comes in a nice looking pouch, although I would have preferred a more airtight container.

Image quality is very good. Vignetting is within normal and acceptable ranges, and it is much better than stacking two separate CPL filters or using separate CPL/ND filters.

I'm just an enthusiast, not a professional. That said, I think this is a terrific filter to keep in the bag because of the overall quality and versatility.