This is probably the best bang for the buck when it comes to lenses. You get "L" like IQ for the price of a kit lens. When the AF got it right, the pictures were SUPER sharp. I did my own comparison against the Canon 24-70 2.8L. A fellow photographer could not distinguish which picture was shot with which lens.
My copy was back focusing so I sent it to Sigma for recalibration under warranty. Unfortunately, that didn't help much as my re-calibrated lens still exhibited the same problem. Although IQ was great if the AF got it right, I found that I was missing too many shots due to the back focusing problem. I sold the lens and bought a Canon USM lens.