I've was given 2 of these and bought one for 18.00, all the newer Ai versions. Astoundingly decent for the price.
My understanding for the older version was that it was good as a portrait/body lens because you could vary the barrel vs pincushion distortion ever so slightly to enhance or reduce.....It was Bob Guccione's favorite lens when he shot Pet's for Penthouse (NSFW!!) I think for this reason.