For me, they wouldn't be worth it, but that's a subjective thing. They're beautiful looking things, and I'm a levergun junkie.
But looking at their 'ballistics with factory loads' page, the gains with the .500 over a more-easily found .454 wouldn't be near enough of a gain to sway me. They show most loads running 2600-3600 ft/lbs; I get up to 3100 with the .454, which is frankly already more than I honestly need for anything.
At roughly $3k by the time magnaporting is added, is it worth it? Not to me personally. On any objective level the amount of the gains don't make it worthwhile, and on a subjective, personal level I have no reason to want a gun in either of their two calibers. But if a guy already shot the .460 or .500 calibers, or if a guy just wanted one because he wanted one, it could certainly be worth it to him.
And if I knew a guy who had one, I'd certainly ask if I could shoot it...