I've read all of Baum's 14 novels and short stories of Oz. I've read books by other Oz authors also. The books are now in public domain and I've read books by today's writers of their own visions. These visions by new Oz writers are often in a "mixed bag", of course.
As for what I think of the original Oz series, I always felt the MGM movie was far better, as well as other versions I saw in other movie versions of the first book or others. Including a cartoon series of a sequel to the movie made in honor of its 50th anniversary. And an anime movie released through Paramount in the 80s, as well as mini-series that makes the first half of the original series into a better developed "series."
The Oz books as a series in their original states can never hold up to the standards of a well-developed series. Baum had way too many inconsistencies in everything from characterizations, to history, to nature of the world's laws in Oz. Right up to the end when he tried to make the Land of Oz like a heavenly paradise for children and escape way from the real world for grown-ups in a literal sense! I kid you not.