It appears that Adorama and B&H started shipping pre-orders of the EF 200-400mm f/4L IS 1.4x lens. The lens has a price tag of $11,799 – just like a small car. Nevertheless, I am sure this lens will sell well with professional photographers.
There are some good reviews about the 200-400mm f/4L IS 1.4x. Canon Explorer of Light
So, how does the lens perform? The copy I had was fantastic. I’m not an empirical tester—I just shot the lens daily at the London Olympics for 16 days on my EOS-1D X bodies, almost always wide open and usually under challenging lighting conditions. I think without the extender the sharpness is a match for any Canon prime telephoto and with the extender the performance is as good or better than any prime lens plus extender combination I have ever used. The auto-focus speed on the 1D X body is on a par with the latest Series II lenses. The image-stabilization allowed me to make a sharp photo of the Queen during the opening ceremonies at a shutter speed of 1/100 of a second at 560mm on a monopod.
The lens is certainly aimed at those who need the ability to work quickly and deftly, and aside from the handling and speed afforded by it, the lens is also capable of producing imagery of the highest quality. The array of low-dispersion elements, fluorite and USD, coupled with the optimized optical structure and lens coatings resulted in imagery that left little to be desired. As with other L series lenses, the quality of imagery, overall performance, and design of the lens is suitable for any working situation, and the added benefit of this lens being able to produce such imagery with the convenience of both a zoom design and integrated 1.4x extender make it truly an effective tool that is ideally situated for sports, wildlife and nature photography.
Finally, for those who want to know the technical background of the lens, there is an article by Canon Digital Learning Center.
The lens should soon be available for rent at Borrow Lenses.