As a child, I made it all the way through The Chronicles of Narnia, and read a couple of the books repeatedly, but I never managed to finish the Lord of the Rings trilogy. As an adult, though, I've rewatched each of the Lord of the Rings movies more times than I like to admit (if TNT airs a weekend marathon of them, I'm a slave to the couch), but I was unmoved by The Lion, The Witch and the Wardrobe, and have no interest whatsoever in the inert subsequent movies, the next of which is forthcoming shortly:
I wonder if the answer to why Tolkien's movies are working while Lewis's aren't lies in this somewhat abstracted paragraph from Adam Gopnik's 2005 essay on Lewis:
Tolkien hated the Narnia books, despite Lewis's avid sponsorship of Tolkien's own mythology, because he hated to see an imagination constrained by the allegorical impulse. Though Tolkien was certainly a devout Catholic, there is no way in which "The Lord of the Rings" is a Christian book, much less a Catholic allegory. The Blessed Land across the sea is a retreat for the already immortal, not, except for Frodo, a reward for the afflicted; dead is dead. The pathos of Aragorn and Arwen's marriage is that, after Aragorn's death, they will never meet again, in Valinor or elsewhere. It is the modernity of the existential arrangement, in tension with the archaicism of the material culture, that makes Tolkien's myth haunting. In the final Narnia book, "The Last Battle," the effort to key the fantasy to the Biblical themes of the Apocalypse is genuinely creepy, with an Aslan Antichrist. The best of the books are the ones, like "The Horse and His Boy," where the allegory is at a minimum and the images just flow.
It seems to me that those writerly sensibilities are matched by those of the filmmakers who took on those competing universes. Peter Jackson was deeply committed to building a complete, coherent world that we could enter entirely, leaving points of reference to our own universe behind because we didn't need them. By contrast, we always enter Narnia through an earlier version of our own world, and Narnia's full of references to it, whether religious metaphor, or tea in a faun's hidey-hole. And the special effects in the movies seem determined to convince us of their miraculousness, not of their reality, it's about refracting our world back to us with new possibilities, rather than about letting us escape into another one.