All these incessant arguments about "IQ", when most people here will never approach the limits of the lenses they have!
Before anybody is allowed to post a lens IQ, colour, rendering, etc comment they should be forced to go see the conditions they are tested in. Bench tests are so far removed from real world use now most of this stuff is irrelevant. If you are using AF then that will have a far greater affect on the sharpness of your images than pretty much anything, assuming you are using two or three times the focal length as a shutter speed, you are on a very heavy tripod, working at one optimal aperture etc etc.
If you are not printing above 20" regularly, if you are using AF, if you are not using a tripod, if you are shooting in anything less than good contrasty light, if you are not shooting wide open, or stopped down, forget bench test resolution figures, they mean nothing.
Oh, and if you want a real dose of reality, ask yourself how this lady does most of this work with a 5D MkII and a 50 f1.8! http://tamarlevine.com/
P.S. After the debacle of Sigma's incompatibility issues, and more importantly, their refusal to stand behind their products and re-chip every single affected lens, I for one, will never buy a Sigma lens regardless of price, features or perceived value. I had a good friend who laughed at me when I got my 16-35 and 24-70, he said his six Sigma lenses cost less, I still use mine and they are worth pretty much what I paid for them ten years ago, his stopped working on his digital bodies and were scrap.