I'd been curious about the story, and a friend loaned me the illustrated version. The story took a while to grow on me. Victoria is awful, and Tristan is a sap for wanting her. Yvaine's tetchiness is certainly understandable, and I don't like how Tristan treats her at first. The royals are a bunch of backstabbers, and the witches are horrible. The whole thing is definitely the dark side of a fairy tale. And I did not appreciate what happens to the unicorn. (I can't help it; my inner little girl still likes unicorns!) But the tension builds and the relationships get more complex, and I came to care about Tristan and Yvaine. The resolution of the witch problem is an interesting twist, and I like what Tristan and Yvaine do after he discovers he's the heir to the throne. I found the ending unexpectedly touching.
Charles Vess' art is very detailed (I don't how long you'd have to look at the market scenes especially to see everything), though at times it has sort of a Beardsley creepiness about it, which is appropriate for the story.
I'm a fan of the movie version, which differs in a number of ways from the book. There are places in which the book is more subtle--which, of course, isn't surprising, given the nature of movies. Although I think the characterisation, probably because the actors are so well-cast, seems more complete in some ways in the movie. I prefer the movie version of Tristan, especially--he doesn't seem as tunnel-visioned. The witch's fate in the book is really more appropriate, but the movie version is exciting. The pirates are much funnier in the movie (and I do not want to spoil the reason why for anyone who hasn't seen it). If you like the movie of The Princess Bride, you'll probably like Stardust.