I saw it and I think it's pretty good as a stand-alone movie, but for a Narnia fan who already has a set view of what the world beyond the wardrobe is supposed to be (and who cannot STAND it being changed for drama's sake), it's really really unfaithful to the book and disappointing in that sense.
NOTE: The content below contains spoilers!
The plot has been changed a lot (much more than LWW).
-Cornelius is captured by Miraz after Caspian's flight. (in the book, Cornelius flees from the palace later and meets Caspian at Dancing Lawn to warn him of Miraz's tracking parties.
-Trumpkin was not sent by Caspian to recieve the four children at Cair Paravel, as it was in the book. He was captured by the Telmarines while they were hunting for Caspian and then brought down near Cair to be executed.
-Caspian was being chased out of the palace (in the book he left in secret, without Miraz's minions on his tail)
-Caspian is dark-haired and speakes with a peculiar accent (he's described as fair-haired in the Voyage of the Dawn Treader and it says nothing about a Telmarine accent, so I don't suppose that I can say the director was wrong to give them that wierd accent. I don't find it so becoming, though)
-Caspian and Susan have a cute little crush on each other. (NOWHERE in ANY of the books is it suggested that there's such a relationship between these two characters. I mean, "You might need to call me again." WHAT?! Totally absurd. Totally just for the sake of having a love interest was that sideplot added , as is the case with many movies. But at least the inflated importance of love interests in LOTR were accurate. I mean, Arwen WAS Aragorn's lover and Eowyn WAS Faramir's lover and Rosie WAS Sam's secret crush. But Susan and Caspian? That just springs out of nowhere.)
-the four children meet Caspian in a skirmish in the woods. Peter actually has a few swipes at Caspian, mistaking him for a Telmarine soldier. (In the book, Peter and Edmund are taken to Aslan's How by Trumpkin just as Caspian and co. are losing faith in the magic of the horn. The boys rush into a council turned sour and help Caspian fight off the traitor Nikabrik, a Wer-Wolf, and a Hag who wanted to call back the White Witch)
-Peter has an heated argument with Caspian about whether they should ambush Miraz at night or just stay in Aslan's How and defend it until Aslan comes along. Peter wins in the end and they ambush Miraz's castle at night. (In the book, Peter and Caspian were always on good terms with each other. They like each other very much. Neither is trying to assert his own authority over the other, and there's no such argument.)
-Susan, Peter, Edmund, and Caspian all go to the stealthy night attack on Miraz's castle, where Caspain screws up the plan by rushing up to Miraz' bedchamber and demanding to know the truth about his father's death when he should have been opening the castle gates for the Narnians below. (There was no such thing in any part of the book. No ambush at all.)
-The ambush was unsuccessful. Peter blames this on Caspian. Caspian blames the whole thing on Peter's idea of ambushing Miraz. They both draw their swords in their tempers, but the arrival of Trumpkin, severely injured, distracts them from actually dueling. (This never happens in the books. In the Voyage of the Dawn Treader, Edmund and Caspian do have an argument over royal authority on the Island of Deathwater. Caspian puts his hand on his hilt but never draws his sword because suddenly Aslan appears on a mountainside and they forget that the argument ever happened.)
-Aslan appears only near the very end (In the book, Lucy leads the other three and Trumpkin to see Aslan before they even meet Caspian. Aslan then gives all of them the breath of bravery, and the boys go to Caspian while the girls and Aslan wake up the trees and waters.)
-Peter offers the sword to Caspian after defeating Miraz in single combat. Caspian cannot kill his uncle, so he leaves Miraz kneeling there. A Telmarine lord then kills Miraz. (This doesnt't happen in the book. Miraz trips over something in the combat ring and Peter waits for him to get up. But then the telmarines rush out, Glostelle kills Miraz and cries out that Peter has treachourously slain Miraz while he was defenseless on the ground. The Telmarines charge forth, and the Narnians quickly join the battle.)
-the children and Telmarines pass through a forked tree trunk. (In the book it is a door without walls attached to it)
-Caspian tries to wake up the White Witch in Aslan's How, under the persuasion of Nikabrik, the Wer-Wolf and the Hag. The Witch appears, trapped in ice and asks for just one drop of a son of Adam's blood to free her. Caspian realizes that this is not what he wanted to call up and struggles, but his palm is already cut (by either Nikabrik or the Wer Wolf, I can't remember)and blood flows out. Then, the White Witch locks him in a seductive gaze and he holds out his hand to her, slowly moving closer. Peter and Edmund and Trumpkin rush in at this moment and fight Nikabrik, the Wer-wolf and the Hag. Peter stops Caspian from giving his blood to the Witch, but then the Witch turns her charm on him and Peter is under her spell. It is Edmund who saves them in the end by stabbing the ice in which the Witch is trapped. (In the book, the Withc never appears because the boys and Trumpkin rush in before the dark magic is actually performed. Caspian was never even tempted to do dark magic and spoke against Nikabrik when he proposed the calling up of the Witch. Peter, of course, was never tempted by the Witch in any of the books.)
-Peter loses faith in Aslan for a while, which is why he proposes the ambush and everything. (although he doesn't support Lucy's proposoal to follow the Aslan he can't see in the book, he doesn't lose faith in the Lion.)
-Edmund seems to be watching Peter's back a lot in the movie. He helps him out in a schoolboy fight in the railway station in the beginning. He stabs the White Witch as she was putting her spell on Peter. He shouts "Stop!" just as the fight between Peter and Caspian is about to break out. (There is very little of such a change in Edmund and Peter's relationship in the books. Peter's the mature leader and Edmund's still very immature in the book. He is still a child in that he sulks and annoys people, but he always backs Lucy up when the others don't.)
So the plot does change a lot, and the characters are really different, too. But the movie is quite good if you don't see it as an adaptation. I mean, a lot of my friends who've never read the books before loved the movie, and I can certainly see why. There's a lot fo fighting, some romance, troubled heros, the god arriving in the nick of time to save the good guys, the bad guys being defeated and then converted to good etc. etc.
Do see it. it's a fair watch.