Without rising star DiCaprio and Mark Wahlberg this would have been considerably more turgid and unappealing. But their charm allows sympathy and involvement with the characters, despite their efforts towards self-destruction.
The movie is unconvincing. At the end, Jim is seen going in through a "stage door," and then we hear him telling the story of his descent and recovery. We can't tell if this is supposed to be genuine testimony or a performance. That's the problem with the whole movie.