Is it much different from the Nikkor 45mm 2.8p ?
IMO very different from the 45/2.8 Ai-P, both because the Voigtländer 40/2 Ultron is one f-stop faster and somewhat wider, producing a very different look in images. In the summer of 2014 it was my favorite walk-around lens on the Df, will post photos once I am sitting by my archive disks. The Ultron gives a nice dimensionality in images but requires stopping down for sharpness. Nice, compact and very fine tolerances. Bokeh highlights are not very appealing stopped down but at f/2 to f/2.8 it paints a very dimensional look that I find very pleasing. Just like the sample photo above, very nice.
I have two, they are both of the older Voigtländer SL silver/black series with Red/Green/Blue stripe on the barrel. The older one has a different type of lens hood than the newer, and the newer of them two is factory chipped. The newest versions of the 40/2 Ultron are SL II (all black, rubber focusing rings) and SL II N (all black, metal focusing rings) - they come with a screw-in close-up lens, giving them limited macro capability. The lens scheme looks identical for SL and SL II, so it is likely that the barrel construction, screw-in hood and close-up lens are the only differences to the older version. Not very expensive, you can find mint copies on eBay for 250-300 and the SL II for 300-400 EUR.