I received it and took it to a NHL game the first night. I've had problems with the Nikon 200-400 in low light, but the Canon was stellar in the same arena. Obviously, having the zoom resulted in more useable images. It is also VERY sharp. This really was the test to see if I would keep it. Based on my evening's take, the answer was yes.
Next day I used it at an NFL game and again, it was very nice having the zoom and teleconverter. It was a day game and the lens was awesome. I also really appreciated being able to reach further down field with the converter. From the sidelines the zoom gave me more usable images since they weren't overly tight as my 400 2.8 would have yielded. Shooting the red zone was much easier also. I now just have a 24-70 on the second camera and the 200-400 on the other.
I really wasn't sure about buying this, but so far it's been wonderful. Times have changed and high end digital cameras like the 1DX can easily produce quality at higher ISO's. Thus the 4.0 versus 2.8 f stop isn't critical in pro stadiums where lighting is good.
I couldn't sense that it was slower at focusing than a 400 2.8. It seems just at fast and maybe better. In fact, my in-focus rate was higher than when I shoot games with the 400 2.8