First, let us stipulate that any movie in which Tom Waits plays the Devil has something going for it. If that movie is directed by Terry Gilliam, it looks even more promising. That being said, "The Imaginarium of Doctor Parnassus," while not quite up to the caliber of "Brazil" or "The Fisher King," does not disapppoint.
Essentially, the movie tells a familiar story of good versus evil. Doctor Parnassus (Christopher Plummer) is a former monk involved in eternal skirmishing with Mr. Nick (Waits). The main action of the movie unfolds as the sixteenth birthday of Parnassus's daughter, Valentina (Lily Cole) approaches. Years earlier, Parnassus had made a deal with Mr. Nick to enable him to woo Valentina's mother: Any child born of this union would belong to Nick as of its sixteenth birthday. The Devil loves to gamble, though; he offers Parnassus a way out: Whichever of them can acquire five souls first will win the soul of Lily.
Assisting Parnassus in his quest is an apparently failed suicide named Tony, and here's where the story gets interesting. "Imaginarium" was the final project of the late Heath Ledger. He plays Tony, but he died before finishing the film. Faced with the prospect of abandoning the project, Gilliam and company came up with a novel concept: The part originally played by Ledger was divided among Ledger, Johnny Depp, Jude Law, and Colin Farrell. In the end, Ledger's scenes occur in what may be thought of as the "real world," and each of the other actors plays a version of Tony within the "Imaginarium": A realm of imagination where people are offered their greatest desires--either somewhat ennobling (if offered by Parnassus) or base (if offered by Mr. Nick).
Thus, the movie ultimately has a great curiosity appeal: How well did the filmmakers pull off this improvisatory gambit? Does the multiple casting work? What would it have been like if the entire role had been played by Ledger (or Depp, Law, or Farrell)? Curiosity, though, can be a distraction from simple enjoyment.
In the end, the movie is certainly worth seeing, especially if you like Terry Gilliam: As with all his films, it is visually beautiful. Unfortunately, the movie's behind-the-scenes story seems almost as compelling as what unfolds on the screen.