I gave up my Nikkor 50mm f/1.8 AI and a Nikkor 35mm f/1.4 AIS and decided to consolidate it all in just the Voigtlander 40mm f/2. Mine is the first version, with the AI coupling prongs still present. The latest version lacks the AI coupling. It is fatter than the 50mm 1.8, about as long or slightly shorter. The amount of barrel distortion is about the same as the 50mm Nikkor, which is far LESS than the distortion of the 35mm Nikkor.
Wide open, the contrast of the Voigtlander is much higher than from either of the Nikkors at the same aperture. The Nikkors suffer from some spherical aberration veiling flare wide open. The CV lens with aspherical element is sharp and contrasty out to the corners.