I have the new version of this lens, I have the: SIGMA 24-70 f/2.8 EX DG Macro DF.
I am VERY satisifed with this Lens. It is incredibly sharp and incredibly cheap. My only complaint is that the focusing is increadibly loud and the lens is very stiff when brand new. The day I got the lens I could barely move the zoom and the focus was very slow. After some time of working the lens in the zoom is much looser and the focusing is much quicker. My other complaint is that the focusing mostor is very loud, I would have rather been charged $50 - $100 more for a USM or HSM.
Summary: 2 Complaints -
1. Very Loud Focusing
2. Very Stiff when new
Everything else about the lens is positive - the price is great and the quality is better. I would say that the results are very similar to the Canon 24-70L. If this lens had an HSM I would have given the overall rating a 10. I suggest this lens for anyone who wants excellent quality on a tighter budget.