I think this lens will be a real hit for wildlife photographers, particularly professionals, or those with sufficiently large bank accounts
While some people compare it to tele primes. While I'd love to own a Canon 600mm f/4 II - I would find the lack of zoom limiting at times - eg for providing habitual context of wildlife, even birds) - or when larger subjects fill the composition. Then other people compare the Canon 200-400mm 1.4x (as as zoom) to xx-300mm 2.8 zoom lenses. However I believe the Canon 200-400mm 1.4x is a lens in a league of it's own, and shouldn't be compared to such lenses.
The closest current Canon lens is the 100-400mm L. But it's design (push/pull), older IS, IQ and ultimate reach are not the same. (For the record I expect the 200-400mm 1.4x lens will have very high IQ, and if the 1.4x inbuilt teleconverter is tailored to the lens, I expect it will be particularly good still @ 500mm f/5.6)
I'm not into sports photography at all (and thus don't need a f/2.8 telelens). Even then, lots of indoor photography requires a faster prime (eg around f/2) So I don't think it should be compared to the Sigma 120-300mm f/2.8 - as they are different lenses.
Now, I won't be the first to say that getting a Sigma 120-300mm f/2.8 and using a 2x teleconverter will be a great 'budget' workaround... (and even then, not THAT cheap, but substantially cheaper than the rumoured price of the Canon 200-400mm 1.4x) However as a dedicated lens, I expect the Canon's AF will be faster and more accurate and the eventual IQ will be better.
I believe the Nikon 200-400mm f/4 lens is also quite a 'comparable' - though of course it doesn't have the 1.4x. (Note: the latest version of this lens was the Nikon 200-400mm f/4G IF-ED VR AF-S, introduced April 2010 - price $7000 US). That's a popular lens with a number of Nikon wildlife photog's.
Although I can afford it, I very much doubt that I'll ever buy the Canon 200-400mm 1.4x- as I'll choose to use and send my money for other things - including donations to international charities. I just can't justify that price (at least not at this time). As I do like to photography birds (both in flight and perched) - as well as other wildlife, the Canon 200-400mm 1.4x offers a lot in terms of zoom range and I'm sure high IQ. The IS will be helpful, particularly at 560mm f/5.6, and f/4 between 200-400 is not at all shabby!
Currently I have the Canon 70-300mm L - which I love for it's amazing IQ and particularly its portability (fits in my Lowe shoulder bag, and is nicely weighted and compact) . On my 7D it translates to a 480mm on a FF, which isn't too bad - though at times I desire more reach (naturally) and of course the 560mm f/5.6 on a 7D would nearly double that - being the equivalent of about 900mm on FF.
Well... that's my 2 cents worth. I'm looking forward to it being 100% released, then the reviews -and more importantly - great photos made with it! Cheers.