I haven't read the book. I watched this movie without any expectations whatsoever, or any knowledge of the plot. Honestly, I didn't even know it was about the legend of King Arthur.
The movie was far better than any of the other Arthur films I have watched. It surpasses Merlin in terms of believability, Camelot in terms of maturity, and almost equals Excalibur. Julienna Marguiles portrays a very convincing Morgaine. The whole story is told from her point of view, and indeed it was a gorgeous retelling. For once, we are given the refreshing view that Morganne was humane, and good. We are given new explanations for the incest between Arthur and Morganne, as well as the blackness of Mordred's character, which is now blamed on Morganne's aunt, Morgause.
What intrigued me about the movie was the portrayal of the Pagan and Christian society, and most interestingly, about how the Pagan goddess still lives on as the Virgin Mary (I still find that interesting, although I don't quite agree with it). Certain things, like having Excalibur return to the goddess and fading off into the silhoette of the Christian cross puzzles me a little to what the symbolic meaning behind it implies. This intermingling of both religions - the link established wasn't as clear as it should have been.
Beautiful performances from Anjelica Huston (The Lady of the Lake), and Joan Allen (Morgause). I didn't really think that Samantha Mathis was suitable to play Guinevere (she didn't really look the part, nor act the part)... and Michael Vartan surprisingly pulled off the part of Lancelot well (Franco Nero is by far one of the most atrocious Lancelots ever). Hans Matheson (Mordred), a relatively new actor, does a better job here than what he did in Les Miserables.
A good cast, and a good movie. I found it a little controversial, and at times slightly disturbing, but it keeps the mystery and intrigue surrounding Avalon as palpable as ever.