The film attempts to fix this by turning the story into a quest - the Isle Of Dreams (a disturbing vignette in the book) becomes a source of evil, powering the various random encounters, and giving the characters more of a reason to pick up Plot Tokens along the way (the swords of the original exiled lords). It works pretty well as a structural change. The problem is that we still don't care that much about most of the characters. To be honest, the movie would have worked better if they'd changed it more. Lucy and Edmund don't get much to do, and if they'd dumped them (almost) entirely from the movie and just had it be about Eustace's evolution from spoiled brat into decent human being then that part of the story (which is by far the most successful - the relationship between Reepicheep and Eustace is very well done) would have had more room to breathe. As it is, the movie feels cramped and muddled. It's ultimately more gripping than The Lion The Witch And The Wardrobe (which I found to be terribly insipid), but I enjoyed it less than I enjoyed Prince Caspian. Much as I enjoyed the books as a child, I think that they're the problem with the movies - hamstringing them in much the same way the first two Harry Potter movies were.
Acting-wise, Caspian, Reepicheep, and Eustace are all good, Edmund and Lucy are improved over earlier films (but still not great). The ship itself looks fantastic, and the CGI is seamless. The action scenes are exciting, and the magic looks magical. I was irked by Aslan's speech at the end, but it's faithful to the book, so it feel churlish that somone put Christianity in my fantasy movie.
The box office figures aren't looking good so far, which is a shame, as I'd love to see The Silver Chair on the big screen. Frankly, I can't ever see anyone filming either The Magician's Nephew or The Last Battle.