The 200-400 is a real investment. But it is also an excellent lens. It's very sharp. Yes it's heavy. handholding is not possible for everyone. 200-400 + 1Dx is about 5kg. So, that really needs good technique to hold that lens. Focus speed on the 1Dx is also very good. I think that my 300 2.8 mk2 is a little faster, but don't have the feeling I will be missing shots by that. AF accurancy is top of the world for the 1Dx and 200-400. AF Speed is at least comparable with the 70-200 2.8 mk2. Background isolation is also very good. Of course the 300 and 400 2.8s might still do a little better. I never do movies with my dslrs, so only stills. I mostly use this lens for sports and wildlife.
I never owned the 100-400 but when I compare with friends with that lens then I love to carry and hold that weight and size. I'm very confident I will get the shot I want. Until now (only 3 weeks I have this beast), it never let me down. Always nice pictures. Will post photos in October when I'm back from safari with this lens.
Concerning price, that is another question. Yes I have the photo I want, yes in a good quality due toe sharpness and AF performance of that lens. Does this mean the price difference is OK. It really depends. I'm only a hobby photo shooter. From the other hand, that lens and a good insurances guaranties me also that the real investment is much lower as 12k because I will get a lot of money when this baby gets sold one day. Money on the bank is nice, as a security. However from financial point of view it is not worth to let your money stand on the bank right now. So, I decided to take that risk, and do enjoy now every day of that lens.