Any "Is it worth it?" question can really only be answered after first answering one of two sets of setup questions.
If this is for a business, how long will it take to amortize the cost differential, assuming the item isn't a base requirement in the first place? Will you make more profit with this purchase as opposed to investing the money some other way?
If this is for pleasure, how much more will you enjoy spending the money on this item than on something else?
And, of course, since even professional artists should derive pleasure from work and the tools used to do the work, both questions might apply even in a business situation.
In any case, the 80/20 rule is a good one to keep in mind, though there are certainly times when it doesn't apply. However, it's been my own personal experience that, if you have to ask the question, it generally does apply.
In the case of the 85s...well, the 1.8 is an awesome lens, and it's damned cheap. And it's the one to go to if autofocus performance is at all a consideration -- if you're looking for something for indoor sports, or fast-paced event (including wedding) shooting, the 1.8 is the better lens.
But the 1.2 certainly has some magical optical properties. If you're looking for the n'est plus ultra with a lens that you're not going to be in a hurry to use, and if you don't wince at spending that kind of money on a hunk of glass and metal, it's certainly worth considering.
Since you're asking the question, I'm guessing it's probably not worth it for you. It's not worth it for me, either, nor for most of those who consider the matter. But it's clearly worth it for neuroanatomist!