Pros: Incredibly funny and well-acted.
Cons: May require a deeper appreciation for weirdness than some viewers can handle.
The usual way to adapt a movie based on a piece of reporting is to tell the story being reported on, leaving out the reporter entirely. With a Jon Ronson book, however, that sort of adaptation doesn't work. Ronson has a unique ability to illustrate a subject's bizarre beliefs and ideas without turning them into nut cases or caricatures, and he does it through recounting his personal interactions with the people he's reporting on. Therefore, leaving out the reporter means losing some of the best parts of the book.
Since the story of a guy interviewing a bunch of ex-soldiers isn't particularly compelling, the screenwriters insert the Ronson character into the film by building an entirely new narrative that serves as a framework for the events described in the book. This main plot of the movie tells the story of Bob Wilton (Ewan McGregor), a reporter who, following a break-up with his girlfriend, goes to Iraq to prove himself. While hanging around Kuwait waiting for clearance to enter the war zone, Wilton meets Lyn Cassady (George Clooney), who he discovers is a former member of the First Earth Battalion, an experimental army unit whose task was to train psychic warriors. When he learns that Cassady has been reactivated and is going into Iraq on a mission, Wilton convinces Lyn to take him along. The fictional tale of the pair's adventures in Iraq is interspersed with flashbacks telling the true story of the First Earth Battalion, from Bill Django's (Jeff Bridges, obviously based on the real world Jim Channon) initial "fact-finding missions" in California to the modern military's use of the "dark side" of psychic warfare in Iraq.
The movie has a very strong Coen Brothers vibe to it. While the inclusion of Clooney and a very Dude-like Bridges no doubt contributes to this, the real similarity is in the film's quirky use of humor. Although the movie is hilarious from start to finish, the actors and film makers keep a straight face throughout and the story itself includes moments of real emotion, a bit of danger, and even a few dark scenes. In addition to most of the funniest scenes from the book, the writers include several Jedi jokes that are not funny in the context of the story but are hilarious to viewers because of McGregor's role as Obi-Wan Kenobi in the Star Wars prequels.
The real strength of the movie is in the acting, especially from Clooney. He plays his character with the intensity of a true believer, but he does it with a subtlety and charm that makes us want to believe that Lyn really does have supernatural powers. The fact that he may actually just be a raving lunatic only crosses the viewer's mind during a few of the more intense and absurd moments of the film. Jeff Bridges also does a great job as Bill Django, though in many of the early scenes he really is just playing a variation of the Dude character from The Big Lebowski. While McGregor's character is the hero of the framing story, his real purpose is to serve as the point of view character and when necessary his incredulousness and frustration reminds us how nutty his subjects really are. Kevin Spacey, who plays the villain of the story, falls back on the smug jerk role that he's played in other movies, but does it well and has just the right amount of screen time to keep from becoming annoying. Stephen Root and Robert Patrick have small roles in the film as another psychic warrior and a good ol' boy Iraq contractor, and both play their roles well.
If you've read the book, you'll be amazed at how well the movie manages to capture the essence of Ronson's story despite adding an entirely new plot. Many of the best scenes of the book show up almost completely unchanged, and most of the ideas in the book are at least referenced in the movie (the only major thing I notices missing was the Art Bell/Heaven's Gate stuff). If you haven't read the book, this is still a very funny, incredibly quirky movie with a neat story and great acting.