I liked the quality of 70-400 so much that I sold my 70-200(sigma) away and bought the 70-400.
I agree about facts of 70-200 discussed earlier. But in my case, I'm going to shoot birds and wildlife generally, for instance bears. And you don't want to go very near wild bears in forest. Even though your aperture would be one stop faster when using 70-200
Considering the quality of f2.8, you lose contrast, and sharpnes. To get better contrast and sharpnes, I used f4.0 and above. And in case of 70-400 I'm going to use f6.3 and above if there is enough light.
Good advantage about 70-200 is its size and weight. You can use it on daily basis, when 70-400 isn't that handy to go around with.
Even though I sold my 70-200, I'm still going to need one. Maybe this time I'll buy sony insted of sigma. 70-400 didn't replace the need of 70-200 in my case. I just need get along with this big one for a while.
Maybe I write some kind of review of the 70-400 'cose I did't find one and hopefully I can write a article about "waterbird shootingraft" that I'm going to build for, goes with out saying, shooting waterbirds.
dslr: Nikon D7000 with some glass. Slr: Nikon FE2 with some manual focusing glass.Www.heikkitaskinenphotography.fi