I tested out one very briefly. I have in the past, owned two differnt Ai 35mm 1.4 manual focus Nikkors. Sold 'em both. My biggest gripe with the 35mm f/1.4 AF-S was strong longitudinal chromatic aberration at wider f/stops, meaning that the lens creates strong fringes of color in front of, and behind, the actual point of sharpest focus. Objects behind the point of focus have a green fringing; objects in front have a sort of purple-ish/magenta-ish color fringing. Unlike lateral color fringing, longitudinal chromatic aberration is NOT, I repeat, is not, correctable in software. Now, some scenes would not show this, but I wanted the lens for use nearly wide-open and at close distances, for situations where this kind of behavior would be very noticeable, and for me, unacceptable. Now, many people do not "see" that issue...it just simply does not register with many people. And, if shots are made in some situations, this defect will not appear. The Canon 85mm f/1.2-L suffers from this same problem, as do MANY otherwise fine,fine lenses.
The Nikkor's strong suit is fast aperture and good background bokeh; the Sigma is sharper than the Nikkor, yes, but has harsher background rendering which is, I think unacceptable for "scenic" or landscape use, or even environmental portraiture use. I didn't feel that I could justify the $1,600 used cost of the 35/1.4 Nikkor, since I currently have a 35/2 Ai-S and a 35/2 AF-D, and I like the AF-D model for its utterly SMALL size and ease of carry, AND for the way it is totally non-threatening in social situations.
The thing about ultra-speed lenses like 50/1.2, 85/1.2, and 35/1.4 primes is that they are BIG. Great big, imposing lenses. I've found that "big-ass lenses" make a lot of people feel tense....they see that monster lens pointed at them, and they feel almost violated...it ruins many pictures of "regular people". people who are used to having a camera pointed at them, ie, photo-people and their friends and families, might be a bit less annoyed.