Heroin is bad. Quitting school is bad. Do heroin, and quit school, and you'll end up a hopeless loser. If you understand those two points, then you already know everything The Basketball Diaries is trying to tell you. If you think the drug issue is any more complex than that, then prepare to be disappointed by this movie.
The movie started off somewhat promisingly, with a look at the lives of Jim Caroll and his friends (as interpreted by the moviemakers, of course). The first part is in turns funny and sad, and is generally realistic enough to be poignant.
As soon as Leonardo diCaprio is forced to play someone spiralling into a nightmare of addiction, however, everything goes downhill. If this movie has any subtle or interesting point to make about drug abuse or society, it forgets it here. It just goes back and forth between silly drug-induced dreams and numerous scenes of petty crime, filthy rooms, and Jim looking increasingly corpselike.
The main reason why this part of the movie seems so absurd is the performance of diCaprio himself. The scenes where he tries to get money from his mother, and where he goes through withdrawal, were so hammily overacted that I stopped being shocked and started laughing. Then I stopped laughing and started wanting to hit the mute button. There is one scene where he seems like he does nothing but moan "maaaaaaaaaa... wahhhhh" for minutes on end. There is a difference between drama and melodrama. It's clear that this movie can't resist portraying the latter at the expense of the former.
Anyhow, if you want a movie that will challenge you to think about the drug issue, watch Traffic. If you want an after-school special, watch the Basketball Diaries.