These are all 100% crops from the center of shots I took with my 20D in Av mode (all apertures shown in table) with no post-processing. I used fine JPEG, ISO 100, Parameter 2 mode, MLU with 2-second timer delay on a tripod, center AF point on fire hydrant ~40m away. All shots were taken outside at midday with the subject in full sunlight (though I was in the shade), except for the Sigma 24mm & 70mm shots, which were taken 24hrs earlier while it was cloudy (sorry!).
After finishing this test I wrote, "I think this new Sigma 24-70/2.8 (complete with the AF fix for my 20D) is a nice lens and a good bargain, but I must admit that I am now rather disappointed in the softness it shows wide open, especially in the longer half of its range. Looking at this comparison makes me wonder if the kit lens and my 50mm prime is all I need after all. I'm guessing I'd have to get a good copy of the Canon 24-70/2.8L to get sharp shots at f/2.8 with a zoom in this range, but that would cost much more than I can afford."
Turns out that I won jStopJojo's auction of his infamously sharp 24-70EX, and as you can see from another test I posted on this site, it is much sharper at the long end. Also, Sigma Japan has agreed to fix the long-end sharpness of the 24-70EX I originally bought or replace it with a new, sharp copy!