I was always an Oz fan, especially of the Witch, and I read it before anyone paid attention to it. Hell, the only people I heard talking about the book were the magazine of Fantasy and Science Fiction who gave it a highly positive review.
When I read it, it quickly became my favorite book of all time. It was seriously that good.
I don't think it ever became an Oprah book, but when the musical arrived and became huge I groaned loudly, feeling like something personal and awesome to me was being taken and turned into something it wasn't.
But the four books, if you disregard how popular it is in the musical audience, are solid, fantastic books. They never dropped in quality though it could be argued Wicked was the best, but none of them were less than "very good".
I can't recommend them enough. Maguire is dark, and funny, and writes beautiful prose and you start to love his characters as much as the classic characters who reside near them.