Having suffered through a mile case of scoliosis myself, I figured that I'd be able to relate to this story of a girl who has to go through the stigma of wearing a brace (although I thankfully never had to wear one). What I didn't realize is the real reason that people read the book.
Given that the book was written in 1973, mentioning masturbation in a children's book was pretty advanced stuff, and discussing it several times made it more than just a gratuitous inclusion.
Again, however, Blume's writing hasn't aged well. Putting aside the references to full-service gas stations and department stores, Deenie and her friends are just a bit too wide eyed and innocent for the 7th graders they are. Phyllis Naylor and her "Alice" series has covered the same topics in the midst of IM and email with a similiar innocence.
The problem I think comes down to putting adult thoughts into children's minds. On one hand, they are portrayed are these innocent creatures, but then they are also fairly technical and clinical about their bodies. It's an odd juxtaposition. Almost as if the author is saying "hey, here are the sexual facts stuff" rather than trying to portray how adolescents understand sexuality.
To some extent, that was Blume's attempt of course -- to give girls (mostly) a safe way to find out correct information about sex, but it makes for a pedantic story. Because, in the end, Deenie's curiosity about masturbation and sexuality really don't have much to do with the rest of the story. One could argue that maybe that just is the way life actually IS, but again I think YA lit has come a long way in 30+ years.