I saw the movie first, and was inspired to read the book. In the middle of my reading I saw the movie again. Even though the movie is nowhere close when it comes to being faithful to Jim Carroll's fantastic novel, I still regard it as one of the most important movies I've ever seen (and Carroll's novel my favourite book).
Even though the movie made up 80% (if not 90%) of Carroll's life, its anti-drugs message is still strongly and powerfully delivered. Unlike "Traffic", the most recent anti-drug movie, "Diaries" is carefully-paced, interesting and most importantly, it speaks directly to the audience. DiCaprio's Carroll (or the film version) is someone who has to sacrifice his dream -- which is to become a basketball player -- to feed his seemingly never-ending hunger to get high. "Diaries" is not only a movie for non-junkies; it is also a movie for drug addicts. DiCaprio's astounding withdrawal scene is the one you're likely to remember out of the whole thing, and I think it provides a mirror for drug abusers to look into, to see how crap they really look.
(mild spoiler) Anyone who's seen this movie should know that drug addiction would pretty much screw up a person's life. Although Carroll -- both the film version and the real-life figure -- came out of it a stronger person, not everyone has it in them to completely kick their habit. Example: Mickey (Mark Wahlberg) has to use his life to pay for a drastic and irreversible action that happened on a whim of fury, when he got cheated by a drug-seller, and Pedro is still on heroin even after reform camp. (end spoiler) "Diaries" is both touching and sad, but more importantly, it's powerful.
Leonardo DiCaprio was perfect as Jim Carroll, if not a little scrawny for a basketball player. I regard Leonardo, along with Joaquin Phoenix, as the finest actor of his generation, and it's not hard to know why. I read a comment here that said he overacted during the scene outside his mom's apartment. Well, I beg to differ. How else was he supposed to act under such extreme desperation? DiCaprio's wailings and whinings were spot-on, 'cause that is exactly how *I* sound when I cry (and I mean, REALLY cry). That scene was the most memorable one for me, followed by his withdrawal scene, which I felt was worth the price of the DVD alone. DiCaprio looked disgusting with drool all over his face which was bloodied earlier on, and it was completely effective. Mark Wahlberg, Patrick McGaw and James Madio all gave strong support, but the star here is really DiCaprio.
To reiterate: "The Basketball Diaries" is one of the most important movies I've seen, and the book is one of the best and also important I've read. Teenagers should see this one. (I'm 15.)