So the characters promote smoking. Big deal. I've watched the movie at least a dozen times and I've lost count of how many times I read the book. I've never even thought about smoking. I know that smoking can cause cancer and kill you, but if an adult who understands the risks wants to smoke, it's nobody's business to tell them they can't. The characters in the movie don't smoke, so the actors and film-makers aren't promoting smoking. Also, the movie is rated R for a reason (namely, sex and language). If you let your child watch this, you need to ask yourself: who's really the bad one here? The people defending the rights of legal adults who choose a behavior, or the parents allowing their children to watch a movie chock-full of sex and language?
The movie is great, full of very good actors and very funny lines. Definitely worth watching, plus I recommend the book.