There can be some sample variation so the quality from some is so-so and the quality from others is excellent. I had one of the excellent ones but rarely used it as I also had a 3.5cm f/1.8 and a 21mm CV lens.
There is a good amount of vignetting and the corners can kind of smear a bit until you stop down past f/5.6 then it is a great lens. Wide open mine was average but at f.5/6 it had a nice balance of the vignette I like and center sharpness I like to draw the eye in to the image. Like I said, I had the killer 3.5cm f/1.8 so I rarely used the 2.8cm.
If you can find one, the CV 28mm has better coating and benefits from a bit more modern production, plus you don't have nearly 50 years of use to contend with which could possibly affect your image quality. Overall the Nikkor is a good lens though and I'd buy another one if I were wanting a 28 on my SP.
Oh yeah, for the price one pays for this lens, it often can't be beat.
Phil Forrest