I've actually had a chance to shoot with this lens. It's ridiculously large, absurdly heavy and the zoom mechanism leaves a lot to desire in terms of speed. Focusing was also sluggish and I didn't really like the fact it requires it's own battery to operate. Because of motorized zoom, I felt like this would be a very good lens for video work, rather than photo and AFAIK that's what it's been mostly used for.
The compression and separation at f2.8 are insane. You can't get that specific look with other lenses, even more so if you use it with a 2x extender, giving you f5.6 at 1000 mm.
Weight is a real issue with this thing, not only to carry it, but to simply move it while shooting requires a significant effort - much more than any of the large telephoto lenses I've used. Even the old Canon EF 600 f4 seems super lightweight compared to this thing, not to mention the mk3 version or the 800mm f5.6. I feel like this lens is best suited at a high end full crew video production, where you have a team of people to help and setup this lens for a particular shot. It's definitively not a one-man task.
Back in the day when it came out it sort of made more sense, because the sensor technology wasn't nearly as good as today and wide apertures played a much higher role. High ISO was considered anything past 1600. Nowdays with almost unlimited ISO ranges, I feel lenses like these just aren't really required. Too many compromises to get that f2.8...