The new 80-400G is a very good optically (as good if not better than the 300mm f/4 prime). It has a zoom/VR which is great.
The only issue I found it is that despite the fact it is a 400mm if often doesn't deliver better reach than the 300mm prime due to a strong focus breathing.
My short experience with the new 80-400G is that when focusing to distance ~5-6m at long end (which is my typical shooting distance for shooting birds, squirrels and other small creatures) I get about the same angle of view as my 300mm prime. Only at distances around infinity it gets closer to 400mm. Therefore the 80-400 designation is a bit misleading. However, this is not uncommon for variable aperture zoom lenses.
if all you need flexibility of zoom and VR and don't mind the price tag, go for it. If you look for IQ then I don't think there is any big difference. If you primarily want the 301-400mm range and shoot smaller subjects, surprisingly, it could be better to stick with 300mm and TC and they'll get you closer...
Personally I still prefer my 300mm f/4D prime with TC over the new 80-400G. But YMMV.