Snarky, 17-year-old, silver-spoon-raised Igby Slocumb has been booted out of every prestigious (and not-so-prestigious) prep school on the East Coast -- and one military academy too. A bored, intelligent, resourceful and willful screw-up, he's almost proud of this record, even though he'd be the first to admit it's a cry for attention.

With a blue-blooded, pill-popping, self-absorbed mother (the hilariously dry Susan Sarandon) dying of breast cancer at home; a materialistically hollow, young Republican brother (a perfectly cast Ryan Phillippe) shining at Columbia University; and an asylum-committed, schizophrenic father (Bill Pullman) who haunts all his childhood memories, Igby (Kieran Culkin) seems to be the only Slocumb sagacious enough to emerge a better person from his sad yet comically dysfunctional family.

So despite the title of this tart black comedy -- "Igby Goes Down" -- its young hero is determined to stay on his feet. He's grown a sardonic, wry sense of humor (if not a tough skin) and become an expert at running away from home. Now, having escaped the limousine taking him to yet another upscale boarding school, he's on the loose in Manhattan, having resolved to get by on his own (or at least with the help of his mother's American Express card), even if he's not entirely sure what that entails.

Sharp, philosophical and zestfully tangy, "Igby Goes Down" is a vaguely familiar but inventively tweaked and poignantly melancholy coming-of-age story that first-time writer-director Burr Steers executes flawlessly and with discerning enthusiasm. Tightly packed with droll dialogue, gloriously glib personalities and crackerjack acting from an under-appreciated cast, the film follows Igby's survival of wits in New York, where he has invited himself to crash at the loft that his flippant, millionaire developer godfather (Jeff Goldblum at his suavely smarmy best) keeps for his saucy, sultry, dancer-junkie mistress (the brilliantly attune Amanda Peet).

"She's a dancer who doesn't dance," Igby muses. "She has a friend who's a painter that doesn't paint. It's kind of like a BoHo version of the Island of Misfit Toys."

With his sunken, expressive, hound-dog eyes and his knack for insightful, empathetic, tormented performances, Culkin ("The Dangerous Lives of Altar Boys," "Cider House Rules") fully embodies Igby's shrewd and sarcastic, gloomy but hopeful, weary and badly bruised soul. The kid's ironic self-awareness of his own self-destructive streak, and his aching desire to somehow untether himself from his patrimonial demons, drive Culkin to admirable, extraordinary extremes.

Another sublime performance comes from Claire Danes, acting again after three years hitting the books at Yale. She is sexy, sullen and equally caustic in the pivotal role of Sookie Sapperstein (I love that name!), a college bohemian who Igby thinks could be his cynical salvation -- until she hops into bed with his perniciously predatory brother.

"We're the same age," she reasons, crying through her locked apartment door as Igby has a breakdown in the hallway outside, knowing his brother will chew her up and spit her out.

But as wistful and pensive as "Igby" can get, Steers never loses sight of his incisive, scintillating comedy instincts or Igby's determination to climb out of the well-bred quagmire into which he was born.

This film is one of those rare, exhilarating cinematic delights that gets even better in hindsight, as you mull over its every nuance in your mind.