Warning: This contains spoilers for those of you who have not read the book and/or seen the movie. Read on at your own risk after consulting your lawyer (and mine).
First off, in general I'd say it's a good movie. There's some parts that made me dislike it, there's some places where they strayed pretty severely from the book, and there's some parts where it was dead amazing. Overall, I ranked it an 'A' on the survey I took.
The movie strays from the book in a few places, most notably in the beginning and later on in a made-up scene and a few other additions. The beginning begins with the actual enactment of the birth of Miraz's son and the flight of Caspian, instead of the landing of the Pevensies on the island of Cair Paravel and Trumpkin retelling the story. In general I do not think this was a bad idea, as it makes more sense (and a more suspenseful beginning) with a third-person omniscient film rather than the book, which follows the actions of the Pevensies. However, they severely played up the pressure behind Caspian's leaving; in the book everyone stops to celebrate, and Caspian makes it out. In the movie, Caspian is only 3 feet from his would-be murderers.
One of the problems with the film is they seemed to play up the suspense when it wasn't that important in the book, and played it down when it was a bigger part of the book. This can be seen when the Pevensies land in Narnia. It takes them all of fifteen minutes to figure out that they're in Cair Paravel and that it's about a thousand years later. As well, the treasure room was, to put it lightly, unimpressive. It showed nothing of the riches of Narnia of earlier times. Instead, it held four chests, one for each of the Pevensies, and that's it. Austere would be a good description.
Another scene they totally made up was a storming of Miraz's castle. It seemed like a quick idea Peter had and wasn't planned that well. There was some suspense, but I felt apathetic to it because I knew that the Narnians would be defeated and have to retreat. They did a nice job with it, and managed to lose Edmund's torch there instead of at Cair Paravel, but it felt rather out-of-place with the rest of the story. There were also some definite Lord of the Rings steals in this scene, most notably when Edmund falls out of the tower a.l.a. Gandalf on the top of Isengard.
After they return again to Aslan's how, I never really felt the urgency to do something. It seemed (to me) that they were still pretty well off. There was never a definite count of their troops, nor a comparison with those of Miraz. They also made the stone table seem small and almost insignificant (Lucy even sits on it). The room is also very well lit; much better than if they had been running low on everything.
The final battle made some diversions from the story as well. Most notable is the partial destruction of the How, and the caving in of the ground under the plain to entrap soldiers. As well, there was an absence of Wimbleweather, the giant, and the battle seemed to still be pretty good on Caspian's side until he nearly gets himself killed and the trees step in. Again, a lack of suspense.
The makers also threw in another subplot (taking out a few original subplots, but I didn't expect them to keep them): a love story of sorts involving Susan. It starts out with a guy in London approaching her. She pretty much avoids him, and then ends up in Narnia. I picked up on the Caspian-Susan thing pretty early, as that's how my life tends to be (very very very subtle indications of feeling...), but didn't expect it to end with a kiss and all. Anyhow, I don't think it's totally worthless, as it underscores the fact that Susan's not the girl of previous books. At the end you see her as a young woman, which is kinda neat. (After all, she is 1300 years older than Caspian...) And I suppose it makes it a movie good for a date as well (not that I'd know, but making a guess...).
The Pevensie's exit to Narnia was a pretty sweet sequence. It transitioned much smoother than, say, chimes and fairy music =]. As well, the animators got to show off their chops at animating live sets (if I'm correct in assuming that was CG).
Overall, the animation was amazing. It's gotten to the point where it's difficult to tell live action from animation, which is pretty sweet. I can't wait to see some future work from them.
As well, I enjoyed the bagpipes played by the mice when they bring Reepicheep before Aslan. It occurred to me that if the mice were men (haha), they would be Scottish. And Trumpkin was Trumpkin, all the way through.
But, the best part of the film was when Nikabrik brings in the hag and werewolf and the hag actually summons the White Witch. The whole sequence is amazing, but I was particularly blown away by the part "Just one drop of Adam's blood..." I felt that it extended the story in a very congruous fashion. In fact, I'd watch it again to see that scene again, it was that good.
This is the first film I've watched since doing a 10-minute film for Rhetoric in a weekend. I was walking out of the theater and it occurred to me just how much work postproduction would be for a film of that size. I definitely have an appreciation for everyone involved in that.
As well, it was a nice touch (to me) that Dr. Cornelius looked strikingly similar to Jerry Garcia. Glasses, hair, beard, build, face, it was all there, except for the guitar. I definitely would not have been surprised to see him pick up Wolf and play some =]
Overall, I'd watch it again. Yeah, it strays from the book, but it's a great story in it's own standing. If nothing else, you should watch it for the bewitchment scene.
[Edit:] After thinking it over and reading another review, I've rated it at 3.5 out of 5. The problem is they took and chopped up Lewis' book and tried to 'modernize' it, and failed miserably.