I've been using this lens for about a month now and I'm very satisfied with it. During this time I only used the lens at 1.2 aperture and manual focus in low light situations. You really need to get some experience with it this way, focusing is very critical, but that's an open door I assume.
I did not come across the focus shift issue, but as stated above, I did not use the lens with parameters that would have effectuated it. And I think I will hardly ever do.
This is the first lens, since I started photography a while back, that I'm really exited about. It opens up a completely new world of possibilities that were impossible with my other lenses that are all f/2.8. And as with almost every piece of technology, compromises are necessary to keep it "affordable", especially with lenses, even the expensive one's.