As anyone who reads more than a few reviews for any particular lens should know by now, you can't judge a lens by a single copy, or even a few copies. Recommendations based on tests of 1, 2, 3 or even 4 lenses are nonsense. Recently I had to try 4 copies of a Canon 70-200 f/4 before I got one that was sharp wide open.
I've had great, good and bad copies of lenses of the exact same type from all the major manufacturers. They key is the general direction of the user reviews. Some lenses are uniformly bad, so don't waster your time. Some are generally good but not great, so if the price is right go for it. Some can be great if you take the time to make sure you get a good one, and a very, very few seem to not have any negative reviews at all (e.g., the Canon f/1.4 35L). The Sigma 24-70 f/2.8 seems to fall into the catagory of "can be great, but be prepared to send a lot of copies back if necessary".
Others on this board say their Sigma 24-70 is "soft" wide open. Mine (on the first try, no less) is the sharpest lens I've ever tested--and I've tested many "L" class Canon lenses, including the 50 f/1.4, 85 f/1.8, 100 f/2, 135 f/2, 24-70 f/2.8, 70-200 f/4 and 70-200 f/2.8. I knew this Sigma was going to be something special when I took a casual snapshot of my daughter and saw the detail in her face: skin pores and strands of hair were easily discernable.
This sharpness of this lens is better than a Tamron 28-75 I just sold (which until this Sigma was the sharpest lens I've ever had), and its much, much better than two Canon 24-70 f/2.8s I recently tried (and sent back). For what they charge for this lens, Canon should be ashamed of themselves.
Color and contrast of the Sigma are not as good as Canon "L" lenses, but they are the equal of Tamron's 28-75. The lens is bigger than the Tamron, but lighter than the Canon. An acceptable compromise to my mind.
Biggest downside for me is the noisy and relatively slow autofocus. Canon and Tamron have Sigma beat here. Don't try to take pictures of your kid's soccer match with this lens unless your prepared for a keeper ratio of 1 to 5 or worse. I have other lenses I'll use for sports.
All in all I'm very happy with this lens. To my mind its the best value in this focal range out there. I give it an "8" overall (for the AF) and a "10" if you consider the price in relation to the competition. Just be patient when looking for a good copy.