What camera do you have? A lot of users seem to report some front focus issues, with occasional back-focus here and there. If you have a camera with micro-adjust - it helps out a lot. If not, you can also send it to Sigma for calibration.
I have a T1i, and I believe that it does not have the micro-adjust. I was using the focus test chart and found out that it has little front focus issue. It's about 5-10mm off.
Would they charge me for the calibration? I heard from users that Sigma might not bother to do the calibration if it's off a little.