Just got camera back from Canon with AF upgrade. Works pretty well but you really have to keep that central square on the subject. To keep subject in focus even outside that box, I tried focussing, hitting AF Lock, then recomposing so subject is off to one side, and it worked well. You can see the result at 0:09 of this video:
https://vimeo.com/91370329
https://vimeo.com/91370329
