Compared this new lens to new Sigma 28-70 EX DG and new Canon 18-55 kit lens. The target was a bookshelf, lots of fine print. The Canon 28-105 had better resolution and contrast at 28, 50 and 70mm than the Sigma at both f5.6 and f8. The Sigma is a real dissapointment at f2.8-5.6 at the 50mm focal length and doesn't sharpen up until f8. At 70 mm both the Canon and Sigma were comparable at the center af f5.6,f8, but again, the field edge was clearly better with the Canon. I returned the Sigma. The kit lens was comparable to the 28-105 at 28mmf8 but couldn't hold up at f5.6. The most pronounced differences were noted at the edge of the field at all focal lengths and apertures. While the kit lens can show good sharpness in the center, it's weakness is the field edge and this isn't helped much even at f8.
haven't compared it to my Canon 50f1.4 yet but my quick estimate is that it will not be too far behind in resolution.
A good purchase