Tested this lens with a Nikon 18-35mm F3.5-4.5 and the Tamron 17-35mm F2.8-4.
The Sigma fared the worst in terms of sharpness and color. Soft wide open, and not really sharp at F8 too. Perhaps the set I tested was a lemon, but it was the case then. Cosmetics and styling looks quite ugly too. Heavy and chunky like most Sigma's.
Being a G lens too, it certainly restricts the user to the newer bodies only.
Good point is that the distortion at 12mm is quite well controlled. Similar to the other two lenses tested.