I have 3, of the older N and N.C radioactive versions. I would be a must replace if I didn't have one. I also have 3 f2 versions and a fine 2.8 version, as well as a ver. 3 Summicron so I can and do compare and contrast
The above comments are very accurate, but sometimes I feel that the truly interesting parts about this lens are sometimes overlooked. It has a very unique optical signature, one that can be used effectively. Its fast, small (especially next to the improvement the G which I just tested) relatively light. I tend to use mine at moderately close in distances, available light, with middle f-stops on my modified aperture ring, around in-between 2.8 and 4, gives to me the best balance of sharp and bokeh. I have repaired and tested a number of Ais versions which have optical differences, and I prefer the older ones since I do mostly B&W, in color the older ones have the yellow cast (one of mine is going thru the de-yellowing UV treatment) which can be nice at times but a bit of a drag to color match. The newer Ais version seems to me to have a sharper, crispy look while the older has more of a fuller, robust look to the negs.
BTW if you can use the newer G, it is really a fantastic lens, I was skeptical but yes it actually is an improvement on the older lens while maintaining it's own unique look. Pricey yes, but you won't be lacking in the preformance area.