I use one on a Canon 6D, and i think it's a stellar performer. Maybe i'm not as picky as other fellow forumers, but my copy didn't need AFMA, and the AF is accurate, though not fast. Bear in mind that i always take photographs of static subjects, and that i always use the central focus point.
I even did a shoot with AI Servo enabled on the 5D MkII and the keeper rate was excellent! All wide angle shots in the following series were shot like this: