Well, I'm in the middle of Witches Abroad right now, and I have to say, I think Saddam's criticism is valid in the case of the Witches' line of books, to the point that I'm convinced that the writers of Shrek took notes from this book (for example, their idea of a despot running a Magic Kingdom where everyone is always happy or else is one of the major themes of this book). To make matters worse, Pratchett is even treading ground he himself has already trod, given that the first book already satirized travel and tourism, which is a major theme of this one as well. I'm enjoying it, because there are always laughs to be found in a Discworld novel, but this one just feels particularly tired.
On the other hand, I loved Wyrd Sisters, largely because I love Shakespeare, who is heavily lampooned in that one, I guess.