Don't really find it sharp unless stopped down to f8. Prefer Tokina 12-24 which is the sharpest in this 10 to 12mm wide arena. I find that f4 for the Tokina at 12mm is already about f6.3-7.1 for the Sigma. But Tokina's CA is slightly worse. Tokina at f8 is friggin sharp.
But shouldn't be much of a problem with sigma's softness unless you are not shooting on tripod or printing big like S8R or more. It's more for the perspective right?