Several months ago I started to read "Wicked." At about the same time the book hit kind of a dead zone and my life hit a bad zone, and I stopped reading. I finally got back to it and just finished it.
I enjoyed "Wicked" for the most part, especially the last several chapters. It is most definitely not a book for kids (they wouldn't be interested in it anyway, and it has some pretty racy scenes).
If you have never seen the movie "The Wizard of Oz," some of "Wicked" might be confusing. Personally, I would recommend that you 1) See the movie "The Wizard of Oz" (or read the book), 2) See the show "Wicked," and then 3) read the book "Wicked," in that order. The book and the show are only about 20% related, from what I saw. I absolutely loved the show. I loved the book too, but in a much different way. The show is light and fun, with a hint of darkness. The book is dark with a little dark humor. Wicked operates on the assumption that you have seen or read "The Wizard of Oz," making many references to events there, which is why I recommend you see/read that first.
The interesting thing is "Wicked" shows the events regarding Dorothy, the scarecrow, the Tin Woodsman, and the Lion in a much different light, but it will not spoil "The Wizard of Oz" for you. It will make you see it differently in some ways though.