Read the Screwtape Letters as a teen, read the Great Divorce first when I was about 17, then as I was in the midst of my Christian conversion in my 30s, read Out of the Silent Planet, I believe it was, for a SF class, read parts of Mere Christianity in my twenties. Somewhere along the way, I read the Lion, the Witch and the Wardrobe, but I just couldn't get into the Narnia books. Wrong time in my life, perhaps.
But I really liked the Great Divorce - reread it not long ago. I need to reread the Screwtape letters with my older eye.
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 need to revisit C.S. Lewis too. Read (and liked!) the whole Narnia series as a boy, read the "SF trilogy" as a teen ("That Hideous Strength" was chilling -- decades before my personal exposure to "committees"), anbd the rest came later.
It took a couple readings and many years before I was old enough to fully comprehend "The Great Divorce."