This is a great lens. Initially, I was worried about Sigma's reputation for poor QC. I don't normally test my lenses, but this time, I decided to print out some test charts to make sure the focus was ok. I had to MA +16, but after that, everything was spot on.
The image quality is just incredible. Paired with my 5DII, the low light abilities are just awesome!
I had considered the Canon 50 1.2L, but read some reports about focusing issues here too. I figured for that price, it should be perfect, and I would rather take a risk with a $500 Sigma.
I had also tried out the Canon 50 1.4, but was left unimpressed. It was much softer wide open and had a reputation for AF failures.