Dark quirky humor. Reminiscent of "Harold and Maude" is some ways, but less the optimism. Most of the humor is straight-faced sarcasm or angry clever insults. Very few people in this film are happy and only those that are even likeable are only likable in limited capacity.
Still there is something strangely luring to watch, rooting for this poor sixteen year old trying to survive a perfectionist domineering mother (Susan Serandon), and a stiff older brother that emotionlessly acts as her henchman.
The director keeps the pace as sharp as the dialogue, never boring, aptly filmed and acted. Honest and hyper-real in an odd caricature sort of way. While teens are at the age of self discovery, Igby is doing his best work at self-destruction. Probably he'd like to find himself, but he's far too busy not being whatever it is his mother is trying to make of him. Perhaps this is what the film is really getting at, that subtle turning point. When you finally realize you have the power to crawl out from under parental expectations. That place where instead of using energy to rebel against something you're supposed to be, you decide to start moving constructively toward something of your own choosing.
While the drug usage did show consequence (namely an overdose and a human pretty much devoid of self-respect) I have a real problem spending my free time watching an act of statutory rape without consequence.
So basically borderline amusing, but with some content that I personally find objectionable.