I was invited to a Canon event hosted by Cathay Photo in Singapore in January this year, event was to showcase the 200-400 f/4 (1.4x) Lens, a number of Lenses were available for Photographers to view & play with, but no Cards were allowed in the Camera as these were still considered "Prototype" Lenses. The Canon people were very tight lipped about the release date, but quietly they seemed to be Ok on indicating a release date before June of this year, I know that Cathay have at least 20 Pre Orders (Including myself) for this Lens, so I expect when it does release Canon will not be able to meet the demand, this could also be part of why the Lens is still not released, canon wanting to build some reasonable stock.
So we wait, and hope, from my own exercise in January, I believe this will be a great Lens, I have used the Nikon 200-400 on a D800 last year on Safari, and although I wasn't overly impressed with the Lens at 400, it was very good from 200-300 & reasonable at the long end, whole package though was brought down by the D800 Buffer, at 4 frames a second there were long wait periods for the buffer to clear, by the time the buffer cleared, so had the wildlife. Very good files, clear & sharp as long as you nail the focus, but not a Camera for fast action, 5DMK3 murders it I feel, not a patch on the 1Dx (wont be popular comments I know with the Nikonisers that look into this site, but it's only one mans opinion after all).