cliffwang said:
Sigma has refinished this lens in 2010. The new version is much better in IQ and AF. I actually just google the new version of Sigma 50mm. Looks no big issue on AF. Google is your good friend.
I replaced an EF 50 f/1.4 with a Sigma 50 f/1.4 last year. I must have scored a good copy, it's acceptable wide open and the IQ really kicks in just a click down at f/1.8 and gets very very good at f/2. There may be AF issues with some copies, but in part the "issues" may have been negative posts from photographers out of reality with the care required shooting at f/1.4. Some gifted people seem to have the right touch, but I tend to click down to f/2 and see the very similar bokeh qualities with a far higher percentage of keepers than shooting at f/1.4.
Sigma may have improved their QC but it's still worth buying new from a reseller who has a test & return policy if the lens is not up to your expectations.
PW