I don't know if I have a "bad" copy, or if there even is such a thing, but I never use the 50 1.4 below 2.8. I just don't like the results. The 50 1.2L, on the other hand, is very usable at any aperture and I really love it. I was fortunate enough to evaluate it for two weeks and it's high on my list of future purchases.
One way to look at it, the 50 1.2L is kind of like a lesser version of 85 1.2LII. The 85 is sharper, but the 50 is very decent, the 85 has better color, but the 50 is still great, and the 85 is more dreamy at 1.2, but the 50 is dreamy as well. And unlike the 85 the 50 focuses instantly. But if I wanted the magical look of the 85, but wanted to save some money I would certainly pick up the 50 1.2, and eventually own both!
PS, get it here and save money: http://shop.usa.canon.com …uct_10051_10051_275772_-1
Regarding focus shift, if you start reading about it, I shot two weddings with the 501.2L, both ceremonies in fact, and parts of the reception, and didn't notice any problems. I'm sure some versions have the problem, but other copies are just fine.
D800, D700, MB D10, 85 1.4G, 70-200 2.8G VR II, 16-35 f4G VR II, 60 2.8G Micro, SB 910 (x4), SB 900, Radio Popper PX Trans (x2), Radio Popper PX Rec (x3)