The film, set in beautiful countryside ranging from New Zealand to Poland and Slovenia, fairly faithfully follows Lewis' story, with some additions to flesh out a short book and a simple plot. Much of the expansions come in the form of battle scenes, and "too much battle" is about the most common criticism I have heard. Lewis tends to gloss over fight scenes in his books, but in the story, Caspian has been fighting a weeks-long war of attrition against the forces of Miraz before the Pevenseys arrive, so compressing it into one grand battle at the climax isn't all that unreasonable.
However, it still does run long, especially coming after the entirely invented sequence of the Narnian raid on Miraz's castle. Not that that scene is unreasonable. It is precisely the sort of thing that Peter and Edmund, having devoured wartime stories of daring British commando assaults on Axis strongholds, and German parachute/glider attacks on Allied fortresses, would come up with. It also provides an opportunity for some important exposition that might have been awkward elsewhere.
Other major changes that might raise Lewis purists' eyebrows concern a couple of the major characters. The dwarf Trumpkin (played by Peter Dinklage) is a suspicious and grumpy character, as might be expected of a member of a persecuted and hunted people, instead of being irrepressibly cheerful. This gives his character opportunity for growth, and a more realistic relation to Caspian and the Pevenseys.
Having reread the book and the beginning of "Voyage of the Dawn Treader", one sees that Lewis had a pattern of being down on Susan. In the book of "Prince Caspian," she's pretty much of a 'wet sock', supposedly, as Aslan says, because "she has listened to her fears." Both Peter and Edmund rapidly recover their warrior skills, and Lucy is always most connected with Aslan, but Susan never seems to get back any experience of having grown up an adventuresome Queen in Narnia before. The movie portrayal of Susan as a warrior woman may be a bit overdone, since she becomes a veritable "Legolas", never missing in battle and swatting down foemen with her bowstave when they get too close. On the other hand, she heroically covers Lucy's escape to Aslan, and takes her rightful place as Queen and commander of the Narnian archers in battle. I like this Susan much better.
Anna Popplewell does a fine job with what she is given and handles her action scenes well, as do Skandar Keynes as Edmund, and William Moseley as Peter, who has a very well-done duel scene against Miraz (Sergio Castellitto). Georgie Henley is touching and sweet reprising her role as the trusting Lucy.
I predict that Ben Barnes will be THE new screen heartthrob. He is dashing and handsome and all that Prince Caspian should be, and matches his accent to the other Telmarines, who are chiefly veteran Spanish and Italian actors. On that side, very nice work by Castellitto, Alicia Borrachereo as his queen, Pierfrancesco Favino as the honorable General Glozelle, and Damian Alcazar as the treacherous Lord Sopespian.
They were well supported by Warwick Davis as Nikabrik, and the voices of Liam Neeson as Aslan, Ken Stott ("Mystery's" Inspector Rebus) as Trufflehunter, and Eddie Izzard as a not-over-the-top Reepicheep. Tilda Swinton made a cameo (not in the book, but reasonably interpolated therefrom) as the White Witch, and once again succeeded in being the most terrifying thing in the movie.
With all the fighting, there is no blood shown, although the violence is intense and too much so for small children.
Quite enjoyable, and well worth watching.