It did the book justice. It may have even improved on it in spite of (or perhaps thanks to) certain missing elements.
The absence of the pirate comic book? Not felt.
The absence of the pure text bits at the end of each episode of the comic? Not felt (or rolled into the story and therefore not absent.)
The absence of the giant squid? HUGE IMPROVEMENT.
The film had more emotional impact than the book did.
It earned its R rating on all counts. Not a film to take the kids to.
Also, it's probably not a film that somebody unfamiliar with the culture of the late '70s and early '80s will fully appreciate. You twenty-somethings who mercilessly mock the decades of my youth... let me know how you liked the film. Oh, and PULL UP YOUR PANTS. Your decade is going to get its comeuppance, rest assured.