It's no great secret that Piers Anthony's novels are not especially progressive when it comes to gender. Indeed, his first Xanth novel, A Spell for Chameleon, may be one of the most blatantly sexist books I've ever read. But I loved the Xanth novels when I was a kid (I read my first, Ogre Ogre, at eight), and will occasionally buy them to reread when I find them in second-hand bookstores.
For reasons that have never been entirely clear, my childhood favorite was Centaur Aisle, published in 1981. I picked up a used copy last night to read, for the first time in possibly twenty years. Centaur Aisle is the fourth novel in the Xanth "trilogy" (a trilogy which currently numbers more than 27 volumes), and features the usual Anthony suspects: relatively young, intelligent, sexually precocious characters adventuring in a pun-heavy, magic-filled world.
Mr. Anthony, Xanth has never been the problem. I suspect your readers are reacting to your female characters, which include a woman whose magical talent is that she, over the course of a single month, cycles between beautiful and stupid to ugly and smart (Get it? It's a metaphor for menstruation!), a woman who can grow any plant, a woman who can turn men to stone with her gaze, and a woman who can make people better if she believes in them. Get it? These are all aspects of stereotypical femininity! Awesome!
What's more, the argument "X may be bad, but look at Y! Y is worse! So stop complaining about X!" is, well, lame. Especially considering that, in this case, X and Y are both figments of Anthony's imagination, and thus things over which he has complete control. It's certainly Anthony's choice whether or not he wants to make his fictional land reflect the development of gender issues over the last, oh, century or so. But defending his decision by comparing one of his own fictional lands to another is pretty disingenuous.
My point is that Anthony appears, in the preface quoted above, either to have failed to understand the complaint or to have brushed it off as largely meaningless. I realize that the book was written nearly 30 years ago, but it's not like feminism was new and different in 1981. There's really no excuse.
In any event, the best Xanth novels are far and away the first and the third; respectively, A Spell for Chameleon and Castle Roogna. If you must read one, read one of them and skip the rest.
By the way - if you've ever wanted a good, concise definition of feminism, I urge you to read Sarah Bunting's brief essay "Yes You Are" here.