I had the 1.4, but the AF totally fell apart after one wedding season (to be fair, it was like 30k shots on it, so I got my money out of it!). A few years back, I ponied up for the 1.2, and have not been disappointed! I honestly don't think that either are super sharp wide open, but that's not really the point (at least for me, anyways) of shooting at 1.2 or 1.4. I have found the 1.2 to be far superior in other image properties, especially contrast - I can't tell you how many images came out waaay flat when shooting wide open on the 1.4. If you have the money, and like the 50 style (it's my favorite! and the 5d3 handles it so much better than the 5d2 did!), than you won't be disappointed by 1.2!
Also, if money is a concern, I'm not sure if anybody mentioned it or not, but the Sigma 50 1.4 seems to be a pretty legit option. I've not used one, but form what I've seen from other photogs, it's a lot better than the Canon 1.4 version.