I have had no issues with the auto focus using this lens. I use it for portraits, soccer, baseball, family photos. Everything and anything. There is a thread in the lens section I have commented in where I discuss this in great detail.
I love this lens, I sent it in to Sigma and it went to C.R.I.S. where they worked on the lens because I felt it was soft at 2.8. The only time I miss the focus is because of me. I don't know why, but I am very shaky when shooting for some reason and I am constantly telling myself to slow down and breathe when pressing the shutter.
This lens IMO focuses a hair, and I mean a hair slower than the Canon. It is very difficult to tell the difference. The difference was neglible to me in what I saved in $$$$ going the Sigma route instead of the Canon.
Sometimes it will hunt, but that is because I am pushing it to it's limit. Let's face it, if the lighting is bad, a lens will hunt. But that is why we add flash to equation right?
I had no need for IS because I am shooting sports mostly. If I am outdoors, I will use a monopod most of the time. With wrestling, I am sitting on the floor right next to the mat. Sometimes I even get on my belly and shoot from there.
I set up the camera on manual. Flash is on auto. SS is 1/250th (sync speed with the flash), iso 800, and the aperture is 2.8. Sometimes I will custom WB, others I will leave the wb on auto. WB for depends on the gym and the lighting. AI servo and center focus point.
I feel this lens is a great alternative to the Canon. The money saved on this lens can go towards a flash unit.
Focus is fast, very fast actually. It is a great lens for action photography, once you know how to use it. If you want to see more photos, let me know and I will post a few more some time tomorrow.
And search for my username in the lens section where someone was asking the age old question of the sigma 70-200 vs. the canon 70-200. It's fairly recent.