Exposing Scientology has become something of a genre unto itself, and this doesn't really break any new ground. There are the usual stories of coercion, confrontations and creepy interactions we've come to expect from Scientologists. If you've seen Going Clear, the Leah Remini series, or even the South Park episode, you won't learn anything you didn't know.
Still, he manages to keep it interesting. The most unique thing he does is hire actors to play out various scenes, with guidance from former Scientologists - mostly Mark Rathbun, the former "Inspector General of the Religious Technology Center". Particular emphasis is given to correctly capturing the explosive temper of David Miscavige.
The biggest weakness in the movie is that Mark Rathbun is kind of an annoying guy. Whereas most former Scientologists are contrite about their own behavior in the cult, he really doesn't take ownership of his past at all. He continuously complains about the abusive tactics the Church uses against him and his family, but whenever Theroux politely points out that he certainly did similar things in the past, he goes into a snit. Weirdly, one of the things that seems to bother him the most is that the Church doesn't acknowledge what a "big shot" he was.
Not a bad way to pass a couple hours, but not likely to be remembered for long.