I picked one up a few years back. From the start, I liked the lens when using it with a M6 ttl or a IIIf. Very sharp, excellent contrast, and great colour rendition. However, it didn't get as much use as my Elmar-m F2.8. And depending on the amount of room in my camera bag, the Nokton was left at home. I had thoughts of selling it and putting the money toward another lens.
However, since purchasing a M8, I really started to enjoy the quality of the Nokton. It always get priority for space in the camera bag. Now, I rarely use the Elmar-M with the M8.
As for portraiture. Yes, a great lens. But, like many have pointed out, it gives extra contrast. Sometimes, it works in your favour.
Now ask me if I'd sell it. Not a chance...