Originally Posted by
Tannin
Q: Why is it legendary?
A: Because they don't make it any more.
Q: Why don't they make it anymore?
A: Because it's an ancient design originally made for film, with a slow aperture and no IS. It is comprehensively out-performed in every way by modern, vastly cheaper, much smaller, much lighter, much more practical lenses such as the 600/4 and the 800/5.6. Modern gear has gone beyond what it could do: there are about a dozen different lenses around from Canon and Nikon, any one of which you'd rather own and use if you need massive reach, and none of them costing more than $20k.
Q: Is that all you have to do to be legendary? Stop making something?
A: No, the thing you stop making has to be notable in some way, and this monster of a lens certainly was notable. An extraordinary unit indeed.
Q: So it's pretty useless these days and basically boring?
A: No. Sure, it's not much use for photography anymore but anyone with a love of fine machinery has an interest in it, just as with (for example) Heavy Harry, or the Great Eastern, or the Spruce Goose.