There's something about Piers Anthony's characters that gets repetative after awhile.. I like the worlds he puts together for them, and often the overall plot (although that became formulaic after the first few Xanth novels), but, just having reread an Incarnations-series book, I was disappointed. I think it must be hard for authors to provide windows into convincingly different characters while maintaining a single flavour (if they so choose) for their works.
Update:I think, comparing this to Rick Cook's Wizardry series, this is a bit more clear. Anthony is either always placing himself in the most prominent male role in the stories (Stile in the Adept series, most of the male incarnations in that series), or he's writing to reassure geeks that they're awesome (or some combination of the two). As I've probably written before, I enjoy Rick Cook's wizardry series - it's an amusing set of stories that puts programmers (from the "real world") into a medieval environment where their programming abilities directly translate into magic, where they find it easy to "get the girl", and where they don't have a tough time with managers, venture capitalists, or whoever's handling the boring details. The relationships still feel natural (if shallow) in Rick Cook's series though, while Anthony's works have a kind of surreal-and-juvenile feel in how relationships work, almost as if they're a form of therapy. The lead male always is highly idealistic, fairly sexually interested but bashful about it, and a "good person" whose merit just hasn't been recognised by any female yet, until a uniquely perceptive female who's "different" comes along, .. etc etc. Doing this once or twice might be ok, but just like people fixated on certain historical events tied to their ancestry (no, I don't have just one example in mind - this actually happens often with various groups), it gets a bit old, and the "you're really awesome inside" tastes strongly of sycophancy once one learns to recognise the flavour. The difference between Rick Cook and Piers Anthony is that Piers Anthony lays it on very thick, while Rick Cook does it in moderation. Piers Anthony's works also are subtly creepy in a way I can't quite identify yet.