Well, I just saw it tonight. Fantastic. The casting was brilliant all the way around; I still can't get over how good Mr. Tumnus was. A lot of the artistic choices made in the cinematography, soundtrack, etc. were powerfully done. When the two armies are charging for each other, and the score fades out into dead silence in the moments before impact, it sent a chill down my spine.
The book itself was quite short, so to make a full-length movie required expanding the story (not how it usually works). The screenwriters had to emphasize themes in the movie which either did not appear in the novel, or were given short shrift. In particular, the Pevensies were much more conscious of the choices before them, which gave their eventual decision to lead the Narnians much greater impact. In the books, the children were essentially swept along by the action. At no point did Peter say, "I choose to risk my life in battle and become king for the sake of people who need me, but who I have only just met." In that sense, the movie told a much stronger story.
Interestingly, Susan was developed as the voice of skepticism, "logic," and reluctance to get involved. This strongly foreshadows her eventual rejection of Narnia, which is probably where they got the idea. In general, the frictions between the siblings were expanded from the mild tiffs shown by C.S. Lewis into full-blown tense conflict. This also made the story much stronger, and was no doubt needed to keep the dramatic arc moving throughout the movie.
A very quick shot which viewers might miss is the White Witch's reaction to Edmond after he [SPOILER] during the battle. That reaction, when compared to the manner that the Witch carried herself in the rest of the movie, has me convinced that the director is a sheer genius.He took a fairly flat character and made her wonderfully complex, in all sorts of subtle ways.
Me like. Go see now.