The ship is first rate: a right Narnian vessel. The acting is pretty good. Eustace does nasty so well that you are surprised at how convincing such a young actor makes the improvements to his character after his undragoning. Most importantly, the changes to the story were not as severe or as disruptive to the plot as those to Prince Caspian. Dawn Treader is the most episodic of the Narnia books. When reading, this is not a problem, because you can do one episode at a time if you want. For a single film it's more of an issue--hence the addition of the green mist and seven swords motifs to try to gain unity. They could have been explained better, but they were not terribly intrusive. After making a convincing dragon out of Eustace, I can see how the director could not resist using him against the sea serpent. Lucy and Edmund have moments of temptation, but they are not caricatures of themselves the way Peter was in Prince Caspian. Not all of the changes to the plot work well or make sense, but at least they do not alter the basic meaning of the work, like having a Peter who had apparently learned nothing from his previous life as High King and who shows up in Prince Caspian less mature than he was in The Lion, the Witch, and the Wardrobe!
I basically have two complaints, and they are probably related. The first is the tendency to replace some of the best dialog ever written with rather lame and bland (in comparison) speeches. All lovers of the books will have their own list of lines they wanted to hear but did not. Caspian's devastating condemnation of slavery, Reep's rebuke of Caspian for forgetting he is "not a private person," Ramandu's response to Eustace about what a star is made of versus what it is, Reep's setting of his nose to the Utter East: the list goes on and on. How could anyone think that the comparative tripe which replaced this stuff was an improvement in any sense of the word? The answer escapes me.
The second and more serious complaint is a watering down of the spiritual message. It is not lost, but it is lessened. This happens in several ways. One is the addition of several speeches that jar because they are not Lewisian at all but rather have their origin in modern pop self-help psychology. Reepicheep playing a Narnian Dr. Phil to the dragon Eustace with a load of self-esteem-inflating schlock is not just annoying--it interferes with the real message of the book, which is present when Eustace admits that he could not have undragoned himself without Aslan. The iconic moment of the watering down for me, though, is the undragoning scene itself. In the book it is a major climactic moment on which we linger. In the movie it flashes past as just one more in a series of magical and marvelous happenings, not necessarily any more significant than any of the others. Finally, Eustace's description of the experience was consistent with the one in the book, but weaker and nothing near as focused. And so I come full circle to my complaint about the dialog.
I enjoyed the film and would not mind seeing it again. It is not the betrayal of Lewis that Prince Caspian was. It does capture his vision even though it has moments when it is working against itself in doing so. But it could so easily have been so much better, more inspiring verbally and more powerful spiritually, in ways that would not have made it either less cinematic or more sectarian. Lewis fans would have loved it even more, and those who do not know Lewis would have been even more powerfully attracted--or repelled. We know the books have that effect on some. It would have been a risk worth taking, in my opinion.