I haven't read the book upon which the movie is based, so I think I'm able to give a relatively unbiased opinion of the film's merits vs. deficiencies.
Tilda Swinton plays Eva, the mother of the titular character. It's apparent that she draws the suspicion, hostility, and curiosity of the people in town pretty much from the get-go. What's not immediately as clear is WHY. The focal event which has turned her neighbours against her is revealed in a series of flashbacks. The movie doesn't put all of its stock, however, into stringing us along, keeping us in suspense until lifting the curtain on what great tragedy occurred to have this woman impugned and hated. Instead, it tells the story of her relationship with her son in an attempt to shed some light on his development and how he ended up a convicted criminal. Before he is even born, she's ambivalent about having a child. He's a fussy infant whom she resents, then a headstrong toddler towards whom she behaves either passively-aggressively or simply full-on aggressively, even accidentally breaking his arm in a fit of frustration. Racked with guilt, she then becomes somewhat overly permissive with Kevin as his manipulative behaviour continues and escalates. Swinton, whom I've found to be a reliable--but not necessarily "star quality"--actress, never strays into either melodramatic nor disinterested territory, which I thought could be a risk with this character. Swinton portrays Eva's vulnerabilities and strengths in a balanced manner and manages to craft a character to whom the audience is sympathetic (without simply pitying her), despite her evident shortcomings.
John C. Reilly, a favourite of mine, plays her clueless husband. Unfortunately, he seems to have carried over the jaunty insouciance of one of his other characters (Dewey Cox comes to mind), because his complete obliviousness to his son's and wife's mental states and struggles doesn't ring true; only a self-obsessed buffoon could overlook such tension and danger in the home. Ezra Miller, who plays Kevin as an adolescent, delivers a simmering, devil-may-care performance which helps Swinton carry the film.
The film, on its own, is worthy of the praise that has been heaped upon it. One of the major drawbacks is the musical score. Jonny Greenwood's twangy, pseudo-country/folk/bluegrass compositions detract from many of the pivotal scenes, managing to completely sap the suspense out of them and making them unintentionally hilarious. With the sublimity of the film and actors overshadowed by a poor choice of music, the bathos is just too sickening to take.
Taut without ever being overly creepy, explaining while never making excuses, and causing appropriate heartache without headache, the movie's a winner in most respects thanks to great acting...and no thanks to the soundtrack, which I'm sure sounds just fine on star-lit evening at the cabin but takes away from, rather than adds to, an otherwise fine film.