Most people will argue that The Lion, the Witch and the Wardrobe was quite faithful to the books, even though they added numerous scenes. I know a movie is not likely to be a word-for-word adaptation, but even for LWW, I was disappointed at Aslan, who to me, lacked the "wildness" and strength of the Lion in the book. Still, despite the fact that the director reportedly remembered the book as one big battle (and adapted it as such), I thought it was pretty good.
Prince Caspian was reportedly a challenge, just because so much of the book itself was told in flashback. Fair enough. IMO that wasn't any reason to add a very lengthy (and traumatic!) castle raid. Never mind that "Companion to Narnia" by Paul Ford states that Prince Caspian was 13 (Ben Barnes is, er, not). Unlike LWW, I felt like Prince Caspian lacked the wonder and joy of the first film. I did buy it on DVD, but I didn't really care for it so much.
So how's Voyage of the Dawn Treader? I'm actually not sure if it was truer to the books than Prince Caspian, but to me it felt truer to the SPIRIT. It was a pretty good film.
The writers/producers/director worked very hard to add in an overarching plot, which I didn't think was a bad idea, actually, and it was kind of interesting. Unfortunately, I don't think the writers were anywhere up to C.S. Lewis' standards, writing a kind of weird enemy for them to pursue, and not doing a great job of it. They not only changed the order of the book's events, but substantially rewrote the events themselves, added new scenes, a couple new characters, at least one removed, new motivations...and yes, the writers rewrote or removed much of Aslan's dialogue. Not only that, but they glossed over so many events, that I'm honestly not sure if someone who hadn't read the books would be able to follow (and would be very curious to see!). I think it would be much, much more accurate to say that the movie was "inspired by" the book, than that it was an adaptation.
Now, if you've read the book, you can probably remember some of the reasons why Hollywood would not have allowed a book-accurate version of "Dawn Treader" to be made (the last few chapters are not movie-exciting)--but I think it would have been really interesting to see how it went, to portray the wonder and magic the way it was shown in the books. I know C.S. Lewis wrote in a non-cinematic manner, but it would just be interesting.
There were a lot of good things, regardless of the deviations. I read a review or two complaining about Eustace, but I found him a true delight. Granted, I might feel differently if I had (bratty) children of my own, but I thought he acted just perfectly. :) Reepicheep was also great, and he and Eustace had some great interactions. The world itself was nice visually, and there were a few nice male wardrobe pieces, though some of Prince Caspian's outfits weren't so flattering on his tiny waist.... (Nothing really interesting at all for female costumes, sadly.) I thought the sea serpent was scarier and grosser than the Kraken from the Pirates of the Caribbean films.
The end credits were a surprising concession to book fans; stay until the names start rolling, at least!
Theologically, I'm no expert, but I'd almost have to say the Dawn Treader film is more "harmless" than "beneficial." Not to say God can't work through anything, mind you, but I don't think the film itself brings much to the table of Christianity. In fact, parts are kind of against Christianity...Lucy does call on Aslan in the darkness, but once it's overcome, she says "We did it!"
The good news is, IF they actually make Silver Chair, they COULD stay truer to the book, since it's pretty much an adventure story, complete with a climax and everything! Now, as to if they WOULD stick to the book, I have no idea.
And as to if the film will even be made at all? Fox would like to make Narnia into a franchise, but they have these ideas that it needs to be a franchise like Harry Potter and Twilight, where the same characters are present from book to book. (Why do you think the White Witch keeps showing up?) Whereas ultimately, the next three stories (in publication order) could pretty much stand alone. I can see them making "The Silver Chair" but I don't see how they'd do the other three books. Since Jill Pole is my favorite Narnian "kid" and Puddleglum is one of my favorite characters, I'd love for them to make that film.
Or then again, if they DID make another film, they might do "The Magician's Nephew," since then at least they'd have the White Witch for continuity. :/
It is kind of neat how by far the most successful of the films ($291,710,957 domestic/$453,300,315 foreign) not only stayed truest to the source material--but it literally had a Christ figure dying for sins!