Just got mine this afternoon! I'm fairly impressed and will probably be keeping it over the Canon. It has a slight edge in wide aperture sharpness, bokeh is noticeably better, and the build is fantastic. Mine seems to front focus a bit near MFD, which I'm assuming CRIS or Sigma NY can remedy.
A few more questions: If I end up returning the lens to Sigma for calibration, will they look into other potential "issues" as well or do I need to specify anything else I feel is wrong with the lens? It was packaged very poorly by Amazon (the box was sliding around freely inside the packaging), which has me worried (paranoia?) that something may have been damaged in shipping. There's also a bit of dust inside the lens, will they clean this free of charge?
The MF ring is very stiff compared to my 10-20mm and 70-200mm. Is this normal? Will it "loosen up" with time?