The premise of the book is really intriguing. Unfortunately, the author chooses to spend his time writing a novel that is about confusing political machinations rather the than interesting premise. I agree that the novel jumps around far too much - as soon as you are comfortable with the characters it seems that they are suddenly completely different! Characters appear and are important, only to disappear for most of the rest of the novel. I didn't find Elphaba to be especially sympathetic, as I found so many of her actions to be maddening - perhaps if I understood what had happened in the missing years it would make more sense?
I have sympathy for the writers of the musical, who had to take this source material and turn it into a musical with a happy ending. I don't think the musical is great, but at least it didn't leave me with a bad taste in my mouth the way the book did.