A promising review and nice overview of the story from ScreenDaily.com:

Music video and commercials director and graphic artist Mike Mills makes an impressive feature directorial debut with Thumbsucker, a short and sweet adaptation of the novel by Walter Kirn.

Of all the high school pictures at Sundance this year, this is by far the best, treating the teenager at its centre as an intelligent human rather than as caricature or stereotype. It is also the most understated, and its audience will be an adult one, not the kids it talks about.

Already selected for a competition slot at Berlin immediately after Sundance, Thumbsucker will be a critics’ darling, and Mills, who comes from the same school as Spike Jonze and Sofia Coppola, has the potential to follow in their footsteps as a major young American voice.

Distributors will have to rely on critics and the stars in the supporting cast to market it; even with the presence of Vince Vaughn, Keanu Reeves and Vincent D’Onofrio in the cast, it’s not as hip as it is thoughtful and melancholy. Mills, who also wrote the script, chooses not to hammer home the point in obvious MTV-style exposition, instead choosing moments and sparse dialogue to hint at their inner lives. Teen viewers used to Mean Girls and The OC will not get it.

The film focuses on 17-year-old Justin Cobb (a striking, intelligent breakthrough performance by Lou Pucci) who lives in a dreary Oregon town with his parents Audrey (Swinton) and Mike (D'Onofrio), she a nurse in her early 40s struggling with her own doubts and missed opportunities, he a man living with the disappointment of a broken college football career.

Struggling to find an identity of his own while his parents are so preoccupied, Justin gets spiritual guidance from his new age orthodontist (Reeves), but falls out with him when he realises that he has no answers and is just as busy searching for new philosophies himself.

He lusts after his school debating club team-mate Rebecca (Kelli Garner), but, like him, she is awash with insecurities and the two can’t get it together.

He relies on props in his life - much to his father's annoyance, he still sucks his thumb, for example. Soon his debating coach (Vaughn) suggests he start taking Ritalin, the notorious drug which schools in the US encourage for attention deficit hyperactivity disorder. Before long, he is experimenting with weed with Rebecca. But none gives him the self-fulfilment or direction he believes he should possess.

Thumbsucker has no plot as such, winding gently through Justin's final year in high school. It touches on his mother's childish preoccupation with a soap star (Bratt), Justin's whirlwind success on the state debating circuit, sexual experimentation with Rebecca and his first heartbreak, and ultimately his own realisation that everyone harbours the same anxieties and flaws and that is ultimately what makes them interesting people.

Mills displays great maturity for a first-timer and while his restraint doesn't always produce lucid results, it does bode well for him in the future. He elicits fine performances from the entire cast, notably Reeves, who shines as the ever-evolving Perry.