You always have to be careful shooting wide open at f/1.4. There is very little room for error. I used to always centre point focus then recompose, but with this lens I pick the focus point I want to use and the hit rate is a lot higher. It can be fickle, particularly in lower light where there is not a strong contrast difference, but so can most other lenses in that situation. My first copy was a complete dud, it did not front or back focus, it just didn't focus on anything at all. The replacement is spot on but still fickle. The images it produces though are well worth any small shortcomings.
This is how I feel. I think maybe just maybe 1 out of 10-15 shots it may legitimately front focus. But most of the time it is dead on for me. Even at wide open if I am forced to shoot at 1.4. Most of the time I stop down just a bit (1.6-2.0), even when I am going for that look. I just wish Sigma made a 35mm and it was of the same caliber and in the same price range.