Interesting question. I was at a show tonight and chatted up the people next to me about the opening of Moulin Rouge (I saw it last week -- LOVED it) and I HATED the movie. Coincidentally, LAST NIGHT this couple watched the film version (which they had never seen) to see if they wanted to get tickets to the musical, but they BOTH hated the movie, and decided against it.
I told them don't let the movie get in the way of their decision. I would tell anyone NOT to watch the movie -- god bless em, but the lead actors in the film aren't singers and on Broadway, holy smokes are they fantastic singers. Another a crucial difference is the portrayal of the MC -- on Broadway it's Danny Burstein, and like just about everything he does, he finds the humanity in a layered, complicated character, which the film actor didn't (or least the way it was edited he didn't).
I think after it opens the ticket prices are going to go even higher and be much harder to get. I would love to see it again -- it's really quite the spectacle. Don't watch the movie. It pales in comparison.