I shoot sports, and I must admit that there have been times recently when I have wondered about the 200-400 1.4X. I even asked Canon CPS for their opinion on it over the 400 f/2.8 ii.
What did CPS say?
Considering I do sports mainly, they said I should stick with the 400 f/2.8 ii.
I will be trying the 200-400 1.4X and I am sure it is an excellent lens, and for those that shoot wildlife etc then wow, couple that lens with an extra 1.4 or even 2 and you have quite some reach. But for me, at the moment, it looks like the 400 f/2.8 ii will remain king.
I will let you know once I do try it out, though not sure yet when that will be.