Although I'm not in the market for a 5DmkIII, I've been reading with interest about the recent issues with light leaks and that body. That got me thinking about my 50mm f/1.4, a lens that I bought fairly soon after getting my first DSLR a few years ago. At the time, I was perplexed why I would carefully aim the AF, but at wide apertures, the area in focus was slightly different than where I had placed that little red dot. Not always, but sometimes. I thought I was doing something wrong. So, my remedy at the time was to take 10-12 clicks of the same exact comp whenever I was shooting that wide, particularly portraits of my two boys. I found that I was able to get some keepers in there somewhere.
Fast forward to 2012. Now that I have my 5DmkII, and have vastly improved my knowledge of the problem, I'm able to remedy the situation. And not with microfocusing, either. The problem I've come across in this lens isn't inaccurate AF, but inconsistently accurate AF. Had I the knowledge that I do now back when it was still under warranty, I might have sent it back asking for a different one. That may or may not have made a difference; what I'm reading is that this problem is likely model wide. How I've remedied the situation is to turn off the AF, turn on the image preview, and zoom in where I was to focus. From there, I can just spin that little black ring into position. The last time I did kid portraits, the results were amazing. I didn't have one shot where I said, "Dang, that's a nice pose. Too bad I can't use it because his eyes are out of focus." And this was a fairly active 4 y/o boy. It's not a perfect solution, but one that works for me.
So, all of that to say, these companies sometimes put out really good products that either have quirks or design flaws. I guess it's up to you to decide which one yours is. I've learned to live with the problems in my 50mm f/1.4. It is otherwise outstanding glass, especially when you consider the price. Since I can't return it, and I'm not willing to fork out money for a 50mm f/1.2 L, I've decided to just stick with it. It's become a lens I have on there all the time when I'm just out shooting. Like anything else, you have to know your gear, with all of the strengths and limitations it might have.