Just wanted to add my two cents to this thread. I went here this past Saturday before seeing "Some Like it Hot." I wasn't sure what to expect given the mixed reviews here, but I honestly was impressed. I spent two full hours there and could have spent longer, but I had to get to dinner and was also getting tired! I felt there was a lot to see and to read. In contrast to some comments above, I actually was amazed at all of the historical objects they had. Maybe people define "historical objects" differently but it seemed like there was something everywhere I looked, even in the gift shop! The museum is set up chronologically and in the very first room they had a cigar box from "The Black Crook," one of the earliest musicals to be performed (1866). You can't get much more historic than that.
There were a few things which I found less than ideal, although they didn't ruin the experience for me:
1) There is SO MUCH text on the walls in almost every room. Had I stood there and read every word, I think I would have been there for six hours. Most people just glanced at this and walked on. I thought these parts were interesting, but it was just too much.
2) There's music playing in every room and the sound often bleeds from one room to another. Maybe younger people (I'm 63) don't even notice this, but I found it distracting.
3) There are not enough places to sit down! (Again, I'm 63) 
4) There's so much to see that by the time I got down to the lowest level (you start at the top and walk down - there is an elevator for those who need it), which is devoted to "the making of a Broadway show," I was too tired to do it justice. It's a lot to absorb. I'm glad they have this section but I would really have to go back and start there (which doesn't really seem cost-effective) to get the most out of it.
My favorite room was the one for "Rent" - if you go, take a look at the wall of flyers and reproductions of some of Larsen's notes. There's the famous one (I believe the original is in the Library of Congress) where he worked out how many minutes are in a year
Even though it's a reproduction, it still gave me goosebumps. They also have original costumes from the production (Mark, Angel, Roger and Mimi). I'm pretty sure I saw them once at the Museum of the City of NY, but it was fun to see them again. I also thought the staff were very cheerful and helpful. Oh, if you have a coat and/or shopping bags you can leave them in lockers in the lower level (not mandatory). They wouldn't be big enough for a suitcase, though.
I was kind of grumbling about the cost before going, but given all there is to see, and the fact that some of it goes to Broadway Cares, I felt it was worth it. I just think if I want to go again, I'm going to wait for a discount, since I'd only be trying to see certain sections that I skimmed over the first time.
Maggie-the-schnoodle