Before getting my 5D III and 24-70 L III had been tempted by to do to Nikon and buy the D800 and 24-70, switching all my lenses across just the same. I desperately wanted the DR I am used to with film, when factoring in the dev and printing flexibility.
I am very glad I did not, because what the 5d III lacks in outright sensor performance, it makes up for with responsiveness and great all round utility. The new lens is a good deal shorter than the Nikon and I did not feel comfortable with such a long standard zoom. That might sound silly, but with the incredible performance of the new lens, I will be well set for higher res cameras of the future. I have little doubt after all the hoo-ha, Canon will pull a finger out re DR and banding in the next full generation leap of sensors.
I don't think the new 5D III will cut it sensor wise for the documentary work I do (often in extreme contrast), but it will for weddings and the like. In the meantime, that incredible 24-70 II will do nicely on my Eos 3s