Used this many years ago when I borrowed lenses from Yashica (was selling cameras) and was impressed. The slip in filters were convenient and saved lots compared to full size filters.

Resolution was very good for a mirror lens with sharpness across the field. contrast good. Build was very solid (metal).

I Would like to find another sample but costs have gone unrealistically high.

I have also used a russian 500mm mac and a korean (I think) 500mm mirror. The mac was good but not as contrasty as teh Yashica (as the mac was not front coated) and the korean sample was useless. I just couldnt get a sharp image even at 1000 shutter speed.