Frank Baum creates really beautiful characters. They are fun and interesting, and (since in Oz anything can come alive) they are often made of common household objects. This makes everyone in Oz have a unique way of functioning and every creature may very well be one of a kind.
About five years ago, when I found out there was a series of Oz books, I wanted desperately to read them. After reading just two, I feel disappointed. Baum has spent so much time on his elaborate cast, that every other aspect of the book suffers. He tends to twist his characters as well, as if he realizes that their characteristics aren't going to fit well in a story, so sometimes things are true of a character and sometimes not.
Baum sends a huge group of characters to look for Ozma. Betsy and Trot particularly get lost next to Dorothy and Button-Bright. The little interaction we get from the characters is not really enough to give us more than a scrap of action or interaction. The plot drags you from one place to another, with little or no connection except that Baum wants to introduce more characters and new lands. Finally, the climax is pitiful. The bad guy doesn't know he has been bad. They turn him into a dove, he sees the error of his ways and repents.