There's a lot to fear walking into "Katy Perry: Part of Me," the latest teen-oriented 3-D concert documentary to hit theaters. Will the film end up just being 90 minutes of self-congratulatory clips that portray the singer as a musical messiah (also known as the Glee 3-D concert approach)? And how often am I going to have to listen to "Firework" during the proceedings?

The answer to those questions is both yes and no. "Part of Me" features the kind of promotional content that makes the movie feel more like an ad than a film, but it's also startlingly honest and revealing. It may be PR, but it's PR wrapped in a relatively interesting disguise.

The documentary follows the candy-crazed pop star as she embarks on a massive year-long world tour. At first, directors Dan Cutforth and Jane Lipsitz focus on the creation of such a grandiose concert, as well as Perry's difficult journey to the spotlight, which involved her transformation from Christian pop star to über-sexualized pop icon.

As the tour drags on, however, the drama turns toward Perry's failed marriage with comedian Russell Brand, who surprisingly makes several appearances in "Part of Me."

Much like other recent documentaries about tween idols, including last year's "Justin Bieber: Never Say Never" and 2009's "Jonas Brothers: The Concert Experience," the film will be most enjoyed by Perry fans. If you don't like her music, "Part of Me" probably won't send you on an iTunes binge afterwards. In fact, the musical highlight of the film has nothing to do with Perry's songs; it's a montage cut to the tune of M83's "Midnight City."

Unlike those other movies, however, Perry actually gets to participate in her documentary's story. In "Never Say Never," for instance, Bieber barely interacted with the audience or talked at all; that was mainly left up to his manager and his mother. After the end credits rolled, audiences knew Bieber just as well as they did before the film began.

On the other hand, Perry is remarkably candid about her life's successes and failures. The singer talks openly about her long-winded journey to stardom, mainly filled with failed recording contracts that attempted to shoehorn Perry into a particular image and sound. One studio planned to hold onto her contract without releasing any of her songs. They didn't think they could sell her music, but they didn't want to risk another company turning her into a star.

The film's most intriguing footage comes as Perry's marriage comes apart on tour, eventually hitting rock bottom before a big show in Brazil. Perry is barely able to talk, much less perform, and at one point, she pulls frustratingly at the ring on her finger. It's a shockingly humanizing moment that Cutforth and Lipsitz show with very little varnish.

The ensuing interviews with Perry don't tiptoe around the divorce and her broken fantasy either. It's a massive surprise that "Part of Me" delves into these dark times as much as it does, and the movie benefits from it as a whole.

For all of the refreshingly honest interviews and footage, however, the documentary can't completely hide that it's, in its essential nature, an advertisement. This means that the audience is treated to several self-congratulatory montages of fans eagerly talking about how Perry has changed their lives. One interview claims that the singer "speaks for our generation," which is hard to take seriously after hearing songs like "Peacock" and "E.T." (the latter of which is used over clips of Brand and Perry falling in love, making the lyrics about infection, poison and abduction victims even more uncomfortable).

It's unfortunate that those moments have to share the same film as Perry's interesting background story and candid tale of failed love. It's just enough to remind the audience why they were cynical of "Part of Me" to begin with.