I upgraded from a 5D2 with a Sigma 70-200 f2.8 w. 1.4TC to a 5d3 and the 100-400L II... it broke the bank but it's worth it! Also works with the Sigma 1.4TC (f8) for that little bit more reach.
Good to know. The 100-400 II does look amazing I must say, although I'd never replace my 70-200. I need the extra reach for things like motorsport & the Mach Loop.