Everything I've seen suggests in decent light they are more than capable, in tricky light things slow down, i take photos of people in poor light often and at f1.4 so af needs to be top class, that for me is real world. So I'd notice it.
I'd love one as a second camera but I won't be leaving my dslr just yet for a few reasons. I've also spent enough on cameras for now and already have a csc with a couple good primes so im well covered. Perhaps when it's a decent price or in a year when the mk3 is out.
What you gonna do with the 5d3 and 7s?