I bought one of these back in 1973 and still have it. I bought it because
Pop Photo (or maybe
Modern Photo back then), did tests on it and found it to be one of the sharpest telephotos they ever tested. I just pulled it out of my old Spotmatic kit and did some tests with it on my k10d. It resolved very nicely at all f-stops tested. I also have the 100 f/4 macro and the 150 did significantly better at f/4. The compressed file attached doesn't show enough detail, so if you want the large files of these tests let me know & I'll e-mail them to you.
Other lenses tested: 18-55 kit, 50-200 kit, 50/2.8 macro F, 50/1.4 FA, 24/3.5 M42, 55/1.8 M42, 80-200, 28-90, and Sigma 21-35.