I got the Sigma 1.4 recently after a few years of shooting with the nifty fifty. Shooting on full frame, no problems, good focus right out of the box. Overall very good image quality, the 1.8 can't touch it at 1.8 obviously. Also, no more pentagon-shaped bokeh. Haven't had any AF issues - the 5DIII locks it in fine even in low light.
I was calibrating it recently, and noticed that it only needed focus adjustment at 1 meter focusing distance and closer. As I pulled away, it was sharp at 0, so I just kept it at 0 and will be more careful with focus up close. (Edit, I wonder if that's the schizophrenic focus that was mentioned. I should try it at longer distances...)