From a very knowledgable Wicked fan, I will say this...
The book is not for everyone, the musical IS. It doesn't matter if you're a boy, a girl, age 5 or age 99... there is not ONE person I've talked to who's seen the show who has not absolutely loved it. I'm not lying.
The book is darker, and some people love it. I fell in love with the musical first, so reading the book was interesting for me. I liked it - but they're almost like two different stories. I do not compare the show to the book at all. If you love the book, you'll love the show, and if you didn't like the book, you'll still love the show.
Go see it!