Sharp wide open, and razor-sharp from f/2,8 even in the borders. The biggest problem at full aperture, but only in specific lighting conditions, is the purple fringing that occurs in the specular highlights.
If you can afford one, and obviously if you shoot low light, buy it now.
And yes, it is expensive, but not so much: the 28/1,4 Nikon AF costs three times as much, used, and it has been discontinued.
With the same price tag of a used Nikkor alone you may buy a 24/1,4 Canon and a 5D body, both new!