Prince Caspian is far, far more watchable than was the very good The Lion, The Witch, and The Wardrobe. It's darker, wittier, and handles its themes more ably. It's long, but for the most part makes the most of its length as it presents a story that works on many levels.
It tells C.S. Lewis's story, but it recognizes that it must find and exploit themes in that tale that will be meaningful for 2008, and the filmmakers wisely chose the relationship between magic (or imagination) and faith. The children have difficulty trusting the magical nature of Narnia... indeed, the Narnians themselves must be helped to faith again after a millennium of bondage to prosaic, political, power-hungry Telmarines, who are made very interesting in this film. The movie aims at the older child, and touches lightly on issues of adolescence: the loss of faith in the imagination, the hormonal anger in Peter and the awkward, innocent sexuality of Susan that partly explain why Aslan finds them too old to return to Narnia: they have more mundane things to think about, and it's time to let go of the magic, or hopefully to incorporate it into their lives.
The film drags a bit at the end trying to build up tension around the final battle sequence, but the pacing problems are mild enough. The sub-plots (for instance Peter and Caspian's rivalry) is awkwardly resolved, but it is perhaps true to the spirit of English school-boys, who might make up on the rugby pitch rather than in emotional cinematic scripts.
The child actors are far, far more reliable than they were in the previous movie, and show surprising emotional range: their coaching shows through far less often, but they keep the spirit of comradeship that made them endearing.