This set of ND and polarizing filters for GoPro cameras is a must-have for any serious filmmaker.
In order to have control over the exposure and be able to get a real cinematic look, one has to work with ND filters. There is no escape from that unless your camera has internal NDs, which is not the case with GoPro.
These K&F filters are very well built and employ top-quality glass and very fine and sturdy metallic frames.
Not that when using the filters, your GoPro will lose its water resistance ability. If you submerge your camera while using the filter you'll kill the camera.
That said, the filters do exactly what they are supposed to do, which is to cut off excessive light and help you control exposure without affecting color, hence "neutral" density.

The set also include a circular polarizer filter, essential to control contrast and reflections in the image.

The entire set with all its different ND gradations and the polarizer takes you to a whole different game when it comes to producing a real cinematic look. It allows you to move away from that "video look" by letting you lock your frame rate to 24fps, and shutter speed to 1/48 while keeping your lens wide open and your ISO in control as well. the same thing you'd do with the "big" cameras.

It all comes in two nice hard shell cases with an internal foam bed specially caved to fit all the filters firmly and safely. They also include a few sealed cleaning wipes in the kit.