I bought it more for experimentation than for real need, but I must say that the K&F Concept medium linear prism filter pleasantly surprised me, essentially for its very precise and complete package.
If you like playing with light, creating dreamy or cinematic effects directly in-camera without having to spend too much time in post-production, this filter is for you.

The effect it produces is unique: the lights trail horizontally, creating a kind of "trail" that can be very evocative, especially when working in urban nighttime environments, with point lights, signs, or headlights.

It has that something that immediately gives character to the shot or video, and allows you to achieve a creative mood right from the moment you shoot, without digital artifice.
For those interested in fashion portraiture, it's definitely recommended (although I'd suggest adding a couple of alternative filters I've seen sold by the same manufacturer, such as the "Triangular Prism" and "Central Hexagonal Effect," which I think can be very satisfying for studio photography).

The build quality is acceptable; the glass is optically clear and shows—it doesn't produce any strange aberrations or color casts.
The metal ring is sturdy and screws easily onto the lens, in this case a 58mm (I got it for a 100mm macro f/2.8).
The package also includes three cleaning cloths, which are always useful when working with filters, especially if you're doing street photography or shooting outdoors.

One small detail to keep in mind: the effect is directional, and the filter can be rotated, so you'll want to carefully study the effect you want to achieve so you can direct it at the right time and pose.

Overall, for €35 (which It's very little), if you like to experiment and think outside the box, it's a nice addition to the kit. It's not a filter you'll use all the time, obviously, but when you take it out... it does its job.