I wanted to be generous and give this 5 out of 10 but this film really doesn't deserve even that extra star to rate it as mediocre.
Let's start from the top: The film opens inside the bomber planes of Germans, dropping bombs on London during WWII. It next cuts to a scene of a mother frantically herding her four youngsters (the 4 who protagonists of the film) to their bomb shelter. In this scene, Edmund disobeys and runs back inside so he can grab a picture of his dad, who is fighting the war. There is a close call with a bomb, followed by Peter admonishing Edmund with the line "why can't you do as you're told?" (or something like that). This scene is followed by a long and boring goodbye scene at the railway station.
On the one hand, I like the fact that these scenes place the story in their proper timeframe; essentially, they act as a historical backdrop to the tale. On the other hand, however, they're not precisely necessary. They're nothing that a couple of text stills or voice-over narration couldn't have accomplished. One might argue that they give the film emotional weight (or some such crock), but they really don't, especially since they don't really have any bearing later. In my cynical opinion, they're an excuse to give Peter something to say (the aforementioned line to Edmund) that he will say later (though not in any meaningful way); a popular Hollywood convention, apparently. Also, I might add this is a kid's book. Why the need to open the movie with such grim scenes as the WWII bombing of London? Again, unnecessary.
However, this stands apart from the main story, so no real harm done...that is until they get to that main story. Changes are made, seemingly at random. Presumably they are done to enhance the drama of the story but all they really do is impede narrative flow and create scenes that are simply wrong. For example, there is a scene involving the four children in the Beavers' dam, getting the scoop on Narnia from the Beavers. Before this scene there is a scene with Edmund and the Witch, with the latter showing the former where her castle lies, between two hills, quite a ways away off in the distance. In the Beavers' dam scene, Edmund is shown to look at the hills, still quite a ways a way. Later in the scene, Edmund sneaks off to report to the Witch. His absence is quickly discovered by his siblings, who follow with the Beavers. It is quickly seen, though, that that prodigious distance to the Witch's house wasn't very great at all, as Edmund is soon shown to be at the castle, with his siblings and the Beavers close behind! Did I somehow miss the teleportation scene? I won't even comment on how this deviates from the book; the scene, by itself, is just ridiculous. In another scene, a fox (a character invented for the movie, as is the scene he is in) helps the children escape by throwing the pursuing wolves off their trail. On moment, the fox is telling the children and beavers that he will throw the pursuing wolves off, the next the wolves have arrived and demand the fox tell them where their quarry is. He tells them they went north, but it turns out that they are in the very tall tree towering over them. Again, I must have blinked and missed the crucial scene where the fox sprinkled fairy dust on the pursued.
Then there's the problem of acting. The four children aren't terrible but they're not very good either. And two of them (the elder two) are made out to be whiners. I guess you can't blame them too much, especially as they're made to say and do things not in the book (they might have fared better if they stuck closer to the original text).
But how do others, more experienced actors fare? Not all that well either. Tilda Swinton does not make a very good White Witch. In fact, she doesn't seem to know what to do with this role, underplaying it to the point of distraction. Also, the physical match between book and film is poor. Similarly, James McEvoy, paints a very limp portrait of Tumnus. No real conviction or anything of interest there either.
The other main gripe I have with the movie is its mimicry of the Lord of the Rings movies. Lots of armor and weapons and posturing and clashing of armies. Unfortunately, it's all pretty dull and hackneyed (and, please...beavers in armor?)
Overall, there's not much of a sense of fun about the film. Everything seems either overblown or underdeveloped. Too much effort has gone into making things look real and not enough into making things wondrous. For instance, a key piece of dialog by the Witch (one in which she calls out for the creatures in her army), is absent. It's moments like those that made the book so much fun that are noticeably missing from the movie.
I don't mean to nitpick this film to death and come off like someone who thinks the movie should have followed the book to the letter. That is not precisely so, although I don't think there's any excuse for not doing so in this case. Sure, there are parts of the book that could have used embellishment or even improvement but there was no real need to drastically change or add anything. Wasn't the reason the Lord of the Rings (or book to film) couldn't be adapted faithfully that the books were too long? Well, there's no such excuse here.