5d3 has confirmation on all AF points. So you choose a point and he will help you on that AF point for confirmation, just as you would hope it works.
I have all 5d's (I, II and III) and before I had de 5d3 is wasn't using manual focus lenses, but since the 5d3 I find it doable to use. I still prefere AF, but that is just me. The 5d3 is quite good with his focus confirmation, 10 times better then 5d2 focus confirmation.
whether that is enough to justify the extra 1000 dollars of money....I'm not so sure. For some it might be a dealbreaker, but I think a 1000 dollars could get me a pretty extra lens to use on a 5d2