As conceived, written and acted by Jason Segel—apparently one of the brighter stars in Judd Apatow's blockbuster-sex-comedy orbit—the character Peter Bretter starts out with a lot going for him. He has a job composing music on a top-rated TV series, and for nearly six years he's had a girlfriend—Sarah Marshall (Kristen Bell), the pretty and popular star of the same show. So this guy's not a total loser—of the type Segel himself played in last summer's Apatow-directed hit, Knocked Up. (He was one of Seth Rogen's slacker buddies.)

However, Peter does lose Sarah. She unceremoniously dumps him at the very beginning of Forgetting Sarah Marshall, and he spends the nearly two-hour  running time of this aptly named romantic romp trying to do just that—forget her. But it's not easy; every time Peter hears her name or sees her on TV, he's reduced to a big ball of tears and whimpers. His brother Brian (Bill Hader) convinces Peter to get away for a while—so where does he go? To Sarah's favorite Hawaiian resort. And guess who's also there? Right! Sarah and her new boyfriend, Aldous Snow, a British rocker played by the incredibly appealing Brit comic Russell Brand.

Not to detract from the multi-talented writer and star of Sarah Marshall, but the minute Russell Brand swans onto the screen, Jason Segel is a goner. It's not that Brand is better-looking (well, a little bit, maybe), or that he has better lines (Segel passes those around judiciously). It's just that Brand is so….well, so cool and so riveting in the way he moves and in the deadpan élan he brings to the outrageous dialogue.

Actually, Sarah Marshall is chock-a-block with scene-stealers. Mila Kunis is charming as the luscious but unpredictable Rachel, desk clerk at Oahu's Turtle Bay resort, who proves the perfect antidote for Peter's lovesickness; Paul Rudd shows up as brain-addled surfer dude; Jack McBrayer (“30 Rock”) is a sex-addled honeymooner, and Jonah Hill stays comfortably in his Apatow-defined persona as a prototypical nerd. And Bell wins points for making Sarah, the self-involved TV star, both sympathetic and funny—even when she's being bitchy.

There's no mistaking the theme of Sarah Marshall. Like the other recent hits from Apatow Productions, this one is about a guy—or guys—trying to grow up. Which leads us to a caveat: Hey, enough already with the juvenile sex stuff. It's amusing when Jason Segel takes off his clothes to stand there, starkers, facing the camera as well as his old and new girlfriends (in separate incidents). But there's something sad and off-putting about seeing fully-fleshed-out adults “doing it” in wacko ways just to be doing it in wacko ways. Adult sex can be fun and funny too. Grown-up moviemakers—and Jason Segel seems to be one—should know that.