While I understand the 300/2.8 II is an excellent lens, I can't understand why this lens is being trumpeted without the 200-400/1.4x also being mentioned. At 560mm with the built in extender it is just as sharp as the 300/2.8 II + 2x III and at 300mm is very close. Add in the flexibility to go between 200mm and 560mm and this is a no brainer choice of the 300/2.8 II unless you are photographing sports where you need the bare 2.8.
Personally, I bought this lens over the 600/II. It was a tough choice but the flexibility makes the lens more valuable. I would of course like more reach but it does a decent job with a 2x extender. If I were photographing exclusively birds then the 600/II would have been the better choice but the fact is I photograph a wide variety of things.
Because it is far too heavy for the 300mm + TC fans who are looking for a portable hiking companion and it is too short for the tarzans who want every mm of length with their 600s and TCs and the sharpest combinations.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye