Got this lens several days ago. Found not a few negative reviews of this lens, generally say, it is a really nice build "L" lens besides the price of it.
I didn't find any focus shift problems on my 5D till now.My lens code starts with UVXXXX. But it is kindly hard to focus during dim light situation.
From flickr's users, some of them said that if focus distance less than 4 feet, the focus have a big trouble when you are using the center point focus. I'm lucky, I don't have this issues. LOL~
Besides the price, it is much better than 50 1.4 USM. The sharpness is all depend on your focus skill.
Canon 50mm f1.2 is the unique autofocus lens in the world( never think about the former 50 1.0L of canono, it looks like a monster...), for this point, what else could I say.