Since striking box office gold with 1994's "Dumb and Dumber" and 1998's "There's Something About Mary," writer-director-brothers Bobby and Peter Farrelly have bypassed the predictable, which would have been to continue with nothing but sexually explicit, raunchy humor, in order to teach a form of tolerance about disabilities. 2000's "Me, Myself & Irene," 2001's "Shallow Hal" (their best film, to date), and now "Stuck on You" may feature their share of bawdy and decidedly un-PC moments, but they are primarily good-hearted and well-meaning human comedies with lead characters who are everything from schizophrenic, to obese, to mentally challenged, to even albinos.

"Stuck on You," which tells the story of 32-year-old conjoined twin brothers Bob (Matt Damon) and Walt (Greg Kinnear), is also a comedy, to be sure, but this time the Farrellys' guffaw-inducing humor is more muted and not as funny. Its instances of going way over-the-top as it explores the daily experiences of conjoined twins, such as when the outgoing Walt is having sex while the more shy Bob is trying to chat with his Internet girlfriend on the computer, are mostly obvious and can't get their full comic mileage with a safe PG-13 rating. In sole terms of how many laughs it garners, "Stuck on You" stands as the Farrelly brothers' most uneven and unreliable effort. Where it makes its zippy two-hour running time worth wading through, then, is in its big heart.

Working as short-order cooks where they never fail to have their customers' food out in three minutes (otherwise, as the sign says, it's free), Bob and Walt are loving, emotionally and physically close siblings living on Martha's Vineyard. Bob is a promising athlete. Walt spends his free time starring in local plays. And when Walt yearns for a full-fledged acting career, he and Bob move to Hollywood. By chance, Walt quickly finds himself starring alongside Cher (playing the exaggerated diva persona of herself) on a cheesy television series called "Honey and the Beaze," while Bob doesn't think he has it in him to tell his adoring, well-matched Internet girlfriend, May (Wen Yann Shih), that he is conjoined at the waist with his brother. When Bob and Walt finally decide to go through with the operation to separate them in an attempt at easier lives, they also risk being separated in a different way; because they share a liver, and Bob has most of it, Walt's chance of survival is 50/50.

In its portrayal of two brothers who have known no other lifestyle than being constantly together, and who are later faced with going their separate ways, "Stuck on You" is accurate and often successfully heartfelt. The sibling relationship between Walt and Bob is a three-dimensional, touching one. They are as close as two people can possibly be and they are generous to the other's needs, but they are not above getting into the occasional spat. The film's best scene is also its most gently handled, heartbreaking one. As Walt and Bob are about to go under the knife on the operating table, they tell each other, "I love you," and there is an undeniable sense that while they may gain more chances out of life, they are also about to lose something just as precious.

In what were clearly demanding roles to undertake, Matt Damon (2002's "The Bourne Identity") and Greg Kinnear (2001's "Someone Like You") are exceptional at their physical comedy, and even better at bringing a realistic tenderness to their brotherly bond. Kinnear arguably has the showier part, since his Walt is more of the go-getter, ladies' man variety, but it is Damon who really impresses as the nervous, less outgoing Bob. The fear he expresses about May—whom he is falling in love with—finding out about his disability is felt with particular urgency. It also helps that Damon and sparkling newcomer Wen Yann Shih, as the unsuspecting May, make an irresistibly cute couple worth rooting for.

Less fully realized in the romance between Walt and daffy aspiring actress April (Eva Mendes), although it doesn't seem to have been intended as a major focal point, anyway. The beautiful Eva Mendes takes a break from her recent string of tough girl cop roles (2003's "2 Fast 2 Furious," 2003's "Out of Time") to radiantly play April as a truly likable young woman who hasn't yet been affected with the full-blown cynicism Hollywood has to offer. Mendes is surprisingly funny, if somewhat underused, and what is so very nice about the written treatment of April is the way in which she does not see Walt and Bob as disabled, but simply fun guys to hang around with. Cher, a proven fine actress in her own right, gets in on the fun as she devilishly skews her real-life persona before unmasking deeper, more sympathetic layers in a lovely late scene between herself and Walt. And Meryl Streep (2002's "The Hours"), also playing herself, proves she has it in her to let loose with something more fluffy and broadly comedic than she's used to.

"Stuck on You" actually works better in its dramatic slice-of-life moments than in its comedy, which is more lightly amusing than fully satisfying. Perhaps this time out the Farrelly's might have found it to their benefit to not play certain scenes with such a madcap style, since it spars with the more reality-based material. "Shallow Hal," for example, did a better job of mixing these two elements with just the right measurements. It should also be noted that while the Farrelly brothers have always enjoyed giving their disabled and handicapped friends parts in their movies, this is the first time it has called attention to itself. In many cases, their placement is unnecessary and even inappropriate, acting as a minor hindrance to the narrative flow. Regardless, "Stuck on You" works more often than not. It is an engaging and affectionate motion picture that may have its fair share of problems, but treats its subject matter with surprising adeptness and leaves you with a smile in your heart.