I recently read "The Great Divorce" for the first time. Very interesting, although not exactly Catholic :-). "The Screwtape Letters" is always terrific - I've got it on the shelf and read it at least once a year.
I first read the Narnia books in 6th grade. They are, to state the obvious, children's books! If you enjoy other books intended for 8-12 year-olds, you might enjoy those on another try.
From a Christian standpoint (as opposed to simply being good stories if you like that type of thing :-), their great strength is in bringing a sense of immediacy to great truths such as creation, redemption, and heaven. Plenty of other literature can have this effect as well, of course. The impact depends on the reader and his mood and stage in life, as well as the text.
I am actually a pretty big fan of that type of lit...I still read some of it from time to time (don't you dare try to take my copy of the Wind in the Willows or the Water-Babies away from me!
I will try it again one day.
Speaking of religious related stories in a Catholic vein, I have two favorites: Leaf by Niggle by Tolkien and the story of the Little Jugglar of Notre Dame, which always gives me hope when I feel like I am not worth a lot and just spinning my wheels...