A mirror lens is the easiest and most affordable way to get into super-telephoto photography. It is also very light, and the lens focuses down to 1.5 meters. But there are also some significant drawbacks. At f/8 your viewfinder gets quite dark, making it harder to focus. Due to the design, all light spots in the background show ups as rings instead of discs.
The lens comes with a set of three filters: Skylight, Neutral Density ND 2x and 4x. They mount on the rear side inside the M42 screw-mount thread.
I haven´t had a chance to test this specific lens myself so far. Here´s an interesting link with sample pictures taken with this lens.