Well, I don't have much to add that hasn't already been said, but I've read a lot of Anthony's books, although it's been 20+ years since I did. The first two Xanth books were really good, but then the decline started. You didn't really notice it with the next two, but it was apparent with the third pair. I must've read the first 12 or so. By the time the main character was a baby, things were pretty awful. It was kind of like when John Hughes directed that stupid movie starring a baby wandering around Manhattan ("Baby's Day Out"). Ugg.
The first Apprentice Adept series (Split Infinity...) was really good. As mentioned earlier, it was part sci-fi, part fantasy. Definitely one of my favorite trilogy's. But the second one was like day-old bread. Not too good. Macroscope is probably the best sci-fi book of his that I've read. It's one of the few books that I've ever bothered to read twice, and I liked it better the second time around. Battle Circle was pretty darn good, and the Chthon (sp?) books were ok. Personally, I remember hating the Space-Tyrant series. I'm not sure how many of them I read. I also read a couple of the "Death" series; can't remember the name, but I wasn't too thrilled with it. The Orn books were kind of annoying. All I really remember was that they were about some kind of bubble-creatures on an ocean planet. They sprayed each other. I remember that the sex scenes pretty much blew (that's one thing that Piers was usually good at

).
Finally, he had one sci-fi book that I remember liking, but that had an entire chapter (maybe two) hacked out by his editors right before publication in order to fit his page quota. For the life of me, I can't remember the name, although it was short, maybe just one 3 or 4 lettered word. It was funny, because one chapter ended with the two main characters cornered and in desperate straights (I'm thinking they were in a barn). But in the next chapter, they were on another planet and everything was hunky-dory. It was really weird. At the end of the book, there was an author's note explaining that 30,000 words or so were cut in editting, and that's why the book made no sense. I'd love to know what that book was, and if there is an author's cut out there.
Lately, I've been thinking of re-reading the first two X books, and also the first Adept series. If anyone out there is interested in reading so of PA's books, that's where I'd start. Macroscope is also a worthy read. But you have to cut the cord after that, or else you'll end up like me.
Edit: Wow, the internet is awesome! Been wondering for a long time what the name of that book was. It was Mute (
www.piers-anthony.com/mute.html). I've gotta say, the description sounds pretty dreadful. But I may have to get the restored version anyway.