"While it is not perfect, I'm finding the Sigma 35 to perform close to the Canon 35 L in AF accuracy. A somewhat consistent and quite noticeable front focus problem on both of my 5D Mark III bodies required AFMA to correct. You may want to capture multiple images in situations with especially shallow DOF - which will be closer to MFD (Minimum Focus Distance). Note that the focus-and-recompose technique can begin to fail as shallower DOF is achieved. "
This is what I encountered with my copy, so I returned it.
Big PITA to get many shots in focus at F/1.4, probably worse than the Canon 50 F/1.2L.
It's a great sharp lens when you get a keeper.
It probably needs adjustment and practice to get them...