I read a whole lot of his stuff, mostly in high school and early college. I have to agree with much of the general sentiment that his earlier stuff tends to be better, and especially the beginnings of his series. The first series I read was the original Apprentice Adept trilogy, and enjoyed it very much. And I believe the original intent was to end it there, but he eventually continued it due to pressure from fans.
That seems to illustrate his main problem, as far as I’m concerned. He doesn’t know when to let a good series end.
His self-absorbed author’s notes started really pissing me off, as well. I never bothered with “Letters to Jenny” or “Bio of an Ogre”, as I knew they would both just be much larger versions of the damned author’s notes.
Xanth was good for the first 8 books. Book 9 (Golem in the Gears) was painful. I haven’t read any of them past Question Quest. (I think the next one in sequence was “The Color of Her Panties”, and I find myself not wanting to admit having a book with such a title by even letting it be visible on my bookshelf…)
Incarnations – started great with “On a Pale Horse”. Some others were okay, and I actually liked “For Love of Evil” a bit more than some of the earlier ones. It ended horribly, though, with “And Eternity…”. I thought that one sucked wind.
Some of the earlier single volumes were pretty good. I liked Battle Circle, and Mute was kind of interesting, although I thought it had a weak ending. Macroscope was interesting, too.
Some of the collaborations are okay, others should be avoided. I thought that If I Pay Thee Not In Gold wasn’t too bad. Caterpillar’s Question was pretty lame.
One should definitely avoid some of the novels that he originally didn’t get published, but then later finally published once he became “big”. If you ever read The E.S.P. Worm or Mercycle, you will probably want to gouge your eyes out…