I bought the 70-200 F/4.0 two weeks ago. It is an excellent lens.
I already have the 70-200 F/2.8 VRII. It too is an excellent lens.
I don't always agree with the things that Brian says just as I am sure he doesn't always agree with some of the things I write here.
He is spot on, however, about the use of an F/2.8 lens with low light.
I will soon sell a Nikon 70-300 VR which I bought about 18 months ago thinking I would have a lens that I could take around with me when weight is an issue.
I'll use the new 70-200 F/4 now in those cases. Also the 1.4 TC works on this lens so I'll be able to have a 300 F/5.6 if in a pinch.
The image quality of the 70-300 is at best so so and I only have myself to blame for having purchased it in the first place.
I have been looking at numerous solutions longer than 85mm for low light photography.
Somebody mentioned a new 135 f/1.8 coming out sooner or later to replace the current f/2.0 DC. We'll cross that bridge when the time comes.
An f/4.0 lens is fine for daylight photography even when the weather is bad.
Dawn or dusk or after sundown will require for my needs a brighter (and heavier) lens.
A 70-200 F/2.8 on a monpod or a tripod will probably produce better images than an f/4.0 when there is very little light. Somehow I almost always am having to tangle with low light.
A point was also made about the medium format Hasselblad having lots of pixels that could now be replaced with a D800. I don't see a studio photographer who used to toil with 12 image rolls of film when 36/37 image Nikons were already available adopting a D800 because there are just as many or even more pixels.
Hasselblad users get dreamy eyed talking about their lenses and what they can do with them just as Nikon/Canon users do when talking about theirs.