The best review I've read, of the new 80-400, is the one written by Ming Thein. Here is a pertinent paragraph:
"In my mind, the 80-400 has several alternatives in roughly the same range: the 70-200/2.8 II with 2x teleconverter; the 70-300/4.5-5.6 VR G; and finally, the 200-400/4. The first option is about the same size and weight (including the converter, smaller without) and gives you another two stops if you don’t need the reach; however, the optics are nowhere near as good as the 80-400. The second option is the lightest and cheapest of the lot, but it has a significantly slower T stop and experiences a noticeable drop in resolving power above 200mm. The final lens is even larger, heavier and more expensive; it’s a ‘proper’ supertele in the same all-magnesium, super-high end build and fast AF mould as the rest of the fast exotics (the 80-400 has a lot of plastics in its external components, presumably to keep weight and cost down); you gain a stop, but boy, does your back pay for it."
He also compares it with Nikon's 400, and found very little difference in sharpness, slightly softer, maybe.
My own experience isn't vast, but I've owned a lot of 400s, and similar lenses (including Sigma 120-400 & Sigma 150-500) through the years, and I own the AF-S 70-300 VR, mentioned by Ming, plus Tamron's excellent 70-200/2.8.
To me, the 80-400 balances better than any of these. It focuses fast on both my V1 and my D600, and it is the sharpest long lens I've come across, especially if one weighs in sharpness from edge to edge, and sharpness in the long end. Nothing else I could ever afford comes close!
The complete Ming review can be found here: http://blog.mingthein.com/2013/05/02...#comment-67109