Widely Regarded: Nikkor 20mm f/4 AI

The Nikkor 20mm f/4 AI (and its very close predecessor the K) was the optical successor to the very well regarded Nikkor 20mm f/3.5 Auto. The K was produced from 1974 to 1977 and the AI released for a short period til the start of 1978, at which point the Nikkor 20mm f/3.5 AI had been released. Both the f/4 AI and f/3.5 AI had good reputations but how are they really?