I've used the Sigma for a few years and have been really happy. I just got the 1.2 on Friday and so far it's so close to the Sigma that I'm not sure what I spent the extra money on. I plan to do a full comparison in the near future though. Here are two shots taken wide open. The Sigma is a bit shorter than 50mm, but not as much as these images would suggest. I believe I was a little further back with the Sigma.
My initial impression is that the Canon is definitely better. I'm not convinced that it's 4x the price better.
Both are definitely a step up from the Canon 1.4 when shooting wide open.
EDIT: replacing attachments with larger versions:
« Last Edit: October 29, 2013, 12:40:21 PM by bchernicoff »
6D, Fuji X-E1
Tamron 24-70 2.8 VC, Canon 50mm f/1.2L, 70-200 f/2.8L IS II, 400mm f/2.8L II, 100mm L IS Macro, Sigma 85mm, & 35mm f/1.4's, Rokinon 14mm f/2.8, Canon 2x Extender II, Kenko 1.4x, 430 EX II, Elinchroms