Here is my current line-up:
5D Mark III
70-200 IS II
I don't make money out of photography and I mainly shoot portraits, love subject isolation and thin DoF. I hardly use F4.0 or above (only if I am taking a picture of multiple people). I spend most of my time in the playgrounds or beaches with my family.
70-200 works perfectly fine for portraits, the only problem I see with it is the reach (in that it is not enough), and also at the long end, I'd like my pictures to look sharper, I'm hoping to achieve more subject isolation with the 300 2.8 II and sharper images. That being said, I do NOT do wild life or birds.
I don't see myself ever replacing 70-200 because you still need the zoom 90% of the time, but for those magical shots (similar to why I need my 85L even though I own 24-70 II), I think I need something else.
I'd prefer people who has owned or used 300 2.8 II (or I) to answer although obviously everyone is free to voice their opinion.
EDIT : After posting this, I also realized maybe I should consider 200 F2.0 with a 1.4 converter. Please feel free to comment on that idea as well.