The 15/3.5 is special in many regards. The lens is huge, beautifully built with an exquisite surface finish, and makes an imposing sight when attached even to the pro-calibre cameras. For not-so-close subjects the drawing geometry is nearly perfect and straight lines stay ruler straight. However, there are other aspects to consider. Up close, the CRC system generates a lot of barrel distortion. It flares easily and for many camera systems is almost impossible to focus with any kind of precision.
I have the earlier f/5.6 versions plus the f/3.5 and love them both.
Here is an example of the straight-line rendition (captured on a D800 though);