I saw the preview of the musical in San Francisco, and it is rather different from the book.
Turtleheart is completely missing. Fiyero is a totally different character, and we completely miss the whole part about visiting his wife. Nessarose is crippled as opposed to being armless. And the political/animal rights subplot is severely truncated and simplified.
I found it best to imagine the book as seperate from the musical. The musical is fun and amusing... it's like the lite version of the book. There's some added things that are really cute, and it stays true to what it is. I'm not saying that it's not good, but I'm not sure what people who really liked the book are going to think about it.
'"Contrairiwise," continued Tweedledee, "if it was so, it might be; and if it were so, it would be; but as it isn't, it ain't. That's logic."'
~Lewis Carroll