Short answers are yes and very satisfied.
I shoot a lot of high school sports so that means poor lighting in most locations. I upgraded to this from a 300 / 2.8 IS with a 1DX. The 300 was too short a lot of the time so I knew I wanted a 400mm lens. I was worried about the loss of a stop (from 2.8 to 4.0) but with the 1DX, it hasn't been an issue. It might have been with the 1DIV I had prior to this.
The focusing is at least as fast as my 300 2.8 and the image quality is the same or better. The zoom is a fantastic bonus. I carry a 70-200 on a second body and was constantly switching between the two bodies when I still had the 300 - not any more. I hardly use it now with the 200-400, it is that versatile. AND, when I am lucky enough to shoot day activities, the 1.4 TC is great - hardly any loss of image quality. I find I get a lot more shots with this one lens than I did when using two lenses in combination.
I sold two other lenses and a violin to be able to afford this but I think it was worth it. I am a better photographer than violinist . . .
Thanks Andrew. Your answer was very helpful. I also have the 70-200 and love it.