I'm undecided about what to make of this play. It certainly deals with a very important question: Why did Pope Pius XII never officially protest against the mass killing of Jews during the Nazi regime in Germany? And: What difference would it have made if he had protested?
In his play Hochhuth claims that a) The Vatican knew exactly what was happening in Auschwitz b) They did not protest because of their own political advantage c) Their protest would have made a difference.
I don't have enough insight on the matter to decide whether these statements are true or not. However, I do think that Hochhuth hit an important and up-to-date subject here, which is worth a serious discussion, even though the play was written in the 60s.
It's a very unsettling play, including a lot of horrible (but true) details about the treatment of Jews during that time, both outside and inside of Auschwitz. Mainly it's about Riccardo, a young priest, and Gerstein, a Nazi officer who doesn't actually share the views of the Nazi party and tries to use his status to actually help Jews. Both of them try their best to make important Church members, and untimately the Pope himself, aware of what is happening and want to make them set up an official protest by the Vatican. It's no surprise to find out that they are not succesful.
Generally, I do think the play is well-written and Hochhuth's views about this subject come across. However, its length and writing style make it impossible to be performed on stage without serious editing and cutting. Furthermore, considering the extent of pages full of stage directions and background information about locations, characters and events, I couldn't help thinking that maybe Hochhuth would have done better writing "Der Stellvertreter" as a novel instead of a play. There is a lot of information available, that just could not be included in a stage performance, like characters' backgrounds that are not given in the dialogue but in the stage directions. These directions are rather extensive, not leaving a lot of room for the director.
Conclusion: I consider the piece to be very good. It does make the reader wonder WHY there was actually no comment from the Vatican. Before I read this, I wasn't even aware there hadn't been such a thing as an official protest. However, I would have preferred Hochhuth not to have done it half-play half-novel but to have made a clear decision for one of them. Personally, I think it should have been a novel, in order not to lose important information.