Lars' post suggests he is basing his evaluation on samples he has seen on the web. I own and use this lens (admittedly have not had it very long) so my evaluation is based on my own pictures. Going by this, I would not say the bokeh is "disappointing," but rather than Nikon seems to have tried to strike a balance between foreground and background bokeh. That seems logical for a 50/1.4 lens intended to be used at full aperture, since its DOF is so narrow that many pictures will include out-of-focus areas both in front of and behind the subject.
Originally Posted by Jonathon Delacour
As you can see in this article (scroll down to section 2) it is impossible to have nice soft bokeh both in front of and behind the subject plane. As the author (a Nikon lens designer) states:
"When spherical aberration is left a bit undercorrected, flares surround the out-of-focus background, thereby resulting in a close-to-ideal out-of-focus background.
In practice, however, this approach offers two problems. First, undercorrected spherical aberration can cause a ring or off-axis aberration in the out-of-focus foreground. This means that defocusing cannot be attained simultaneously for both foreground and background."
(He goes on to describe how his boss solved the problem by designing Nikon's "defocus-control" lenses, which allow the smoothest bokeh to be placed either in front of or behind the in-focus plane.)
Unfortunately, most of the photos I've shot so far with the 50/1.4G have been for my employer, so I don't feel I ought to post them without their permission; maybe it would be okay, though, to post just a section of one:
I assume that the background of this image shows the issue Lars mentions: you'll notice that the lettering (actually three-dimensional metal sign letters about 10 inches tall and 1 inch deep) show a structure with some fairly abrupt edges, even though the overall blurring is very smooth. What you can't see in this picture, but which I've observed in others (close-up food shots in which the customer wanted a very narrow range of sharpness) is that the out-of-focus area in front of the focus plane is very smooth and soft.
My impression is that Nikon has made a good tradeoff between desirable foreground bokeh and desirable background bokeh, with foreground objects (which will be more prominent in the image) very smooth and background objects only a problem if there are strongly contrasting edges (such as the metal letters in my example.)
I don't know how this compares to the Sigma lens because I haven't seen or used the Sigma. I just wanted to point out that it's not simply a case of "good bokeh" or "bad bokeh," but rather WHERE you want your good bokeh to be, and the best choice for one kind of photography may not be the best for another kind (which is why it was pretty smart of Nikon to make those two DC lenses.)