I've never read the original, but this stage adaptation of the novel is a very intense and memorable piece that raises a number of interesting questions and ambiguities about good and evil, and delusion and reality. It's difficult to draw exactly a "message" from the piece or even the exact facts of what occurred and what was dreamt, but that adds to the nightmarish effectiveness.
The characters are consistent yet have complexities within the play's one act. Perhaps most pointedly the piece reveals very memorably the dangerous ways that Inquisition-like movements and organizations -- which have been common throughout history -- operate and affect the individual.