May I say up front that were the 200-400 priced at, say, $7000 I would suffer none of the ambivalence I have towards the 200-400. Given a $12k tag, I can't escape having mixed feelings about the 200-400. I do not own this lens but rented the lens for one week from a trusted rental house in Cordova TN. My rental served, in part, as a pre-purchase audition. Our rental lens was used for one week shooting dog water rescue testing in available light. This type of work could work could be likened to shooting agility outdoors, on water and often in high-contrast light. Normally I'd use the 200/2, 70-200 II and the 100-400 to cover this kind of shoot with 1Dx and 1D IV bodies. The 200-400 was used to replace both the 200/2 and the 100-400 during our rental period, and the lens was used only on a 1Dx. I rarely used the drop-in CP.
The 200-400 image quality is good, but not uppermost in the Canon line. I'd describe the lens as sharp without the TC engaged; and sharpness to fall off modestly but measurably once the 1.4 is engaged. I would not place sharpness with the 70-200 II, however. There is little comparison, in my mind, between sharpness and rendition of the 200-400 and the 200/2 or 300/2.8 - but that comparison may not be a fair one. The 200-400 is quite contrasty, and color rendition is vibrant. I'd go so far as to say that the lens *has a look*.
Auto focus speed is good and AF is decisive on the 1Dx, but definitely not what we enjoy on the current 2.8 zooms or large aperture telephoto primes. 200-400 AF and IS are considerably better, however, than the 100-400 as you would expect them to be. The 200-400 IS is excellent, and the Mode-3 tripod IS is a pleasure to use. Several times I unintentionally engaged the internal 1.4, because of what I'd call either a design flaw in the TC-Lock or a possible defect in our rental copy's Lock. Our rental copy appeared quite new, and I have no recent to beliveve our copy was not up to par. The lock was completely reliable in keeping the TC from becoming disengaged, however. Lens weight and apparent bulk are high, but balance is decent with a 1Dx mounted. In the 3500 images I recorded, the was an even distribution of hand-held, monopod and tripod use. I'd need to be able to workup to handholding the 200-400 all day long.
The reason to own the 200-400 relates to the shots you *won't* miss by virtue of its zoom function over this FL range at f4. In the work I do, a subject simply moves out of the frame when a long prime is in use, or is too far away when a shorter FL was loaded. So lots of images are possible by avoiding fixed FL lenses. Clearly this is not a novel thought, and such logic could be applied to any zoom lens - but it's of special meaning here by virtue of what FL are being covered at f4. An internal 1.4 simply expands this principle a notch further, and the many (dirty) steps that are eliminated by allowing the 1.4 to live permanently inside the lens package are meaningful. One can't help but guess that internal TC's will appear in upcoming products. There is talk that this lens *pays for itself* by eliminating two or more expensive prime telephoto lenses from your collection. I really can't subscribe to this argument for my work, but your experience may certainly vary. With this in mind, you may find a 100-400/1.4 external TC quite liberating financially...
All of this begging the question of what dollar value is reasonably assigned to this group of attributes and tradeoffs. Were cost of no concern, I'd certainly buy the 200-400, even given the above caveats. I can live with the weight, image quality and AF with the knowledge that I'll succeed getting images that I'd miss with any other lens or combination of lenses. Cost being a priority, though, has me appreciating the long-in-tooth 100-400 for what it is and what it isn't; and contemplating buying a used Nikon 200-400 (at a bit more than a third the Canon's cost) and a D800.
Trite to say, but what we really need is a modernized 100-400 at, say, under $3500.