This is very much an avant garde 1960's play steeped in pretentiousness and a play that its author labelled as Marxist. It is absolutely a lot of fun and I would love to see it staged however. It uses a bunch of trendy techniques from European play writing in this period, but uses them effectively and with a lot of fun and interesting ideas. The play is a play within a play, but the play within the play is coterminous with the play itself. Much of the play is told in verse. I am also not quite sure how you would stage it with a manageable cast.
I find both de Sade and Marat to be fairly contemptible people, but they are of course complex figures and despite the experimental nature of the play captures them well. The play is about revolution, ideological commitment, the failure of revolutions and madness. The characters in the play with the play are played by insane people within the asylum, so we see not characters but the insane acting them. Its good fun and well written. I was not sure of the writing at first, but it won me over.
It should be pretentious trash, but its not. Plus it has the world's best title.