I have this lens. It's the sharpest lens I own...or at least it's the most capable of giving me very, very sharp photos. Unfortunately, it doesn't focus all that accurately, so using it at wide apertures is hit and miss.
I believe this is a common issue with this lens.
I have heard some say that it's image quality is almost on par with lenses like the Canon 24mm F1.4 L or the 35mm F1.4 L....which are both leaps and bounds above Canon 28mm F1.8.
Some of the newer camera bodies have a feature called micro adjust, where you can dial in specific focus adjustments and the camera remembers that setting for each specific lens. I'm thinking that with that feature, one could overcome the focus issue. Of course, you could send the lens (and probably camera) in for service and adjustment...but Sigma doesn't have a service centre in Canada, so I never tried that.