Hi Folks: Found this video today and thought I'd add it in here. It's pretty impressive, even if it does require it's own sherpa to carry it. Also, I must add that I've always said I'm a photographer, not a cinematographer (that's my son's job), but seeing the video of that Flicker is very nice.
Mike.Hands on with the Canon EF 1200 f/5.6L Lens
I've used this monster of a lens. The short video under-sells just how challenging it is to use. At 1200mm you'll need a secondary spotting scope and a spotter to have any clue what you shoot, and where to point the camera. Otherwise your shooting will consist of 90+% of looking for the subject with the viewfinder - the FOV is so narrow eyeballing it becomes an exercise in frustration. It might be easier with small subject matter which is visually separated from the background, though, such as the woodpecker in the video.
Also, the lens is very susceptible to vibration and mirror slap, and proper tripod and shooting technique are more necessary than with probably anything else out there.
It was great to see video out of the lens, haven't seen that before - looks stunning!