its absolutely exceptional Julian!
I used to use the nikon 50mm 1.4, but got sick of how soft it was at 1.4 at distances. It basically made it useless in low light if I was any distance beyond about 2 meters from the subject. The sigma is not only sharper at 1.4 it is probably shaper than at 1.4 than the nikon is at any aperture.
at f4 its the sharpest, highest contrast lens I have ever used of any focal length. Im pretty sure it out resolves the D800
In fact the only thing I have seen wrong with it is some very minor spherochromatism as can be seen in this photo.
Bugger all distortion, or any other aberrations that I can find.
Generally, its awesome!