They are really the kinds of books that are best enjoyed as children - I never got the Christ allusions until I was 20 and read Lewis's essays. Like Enid Blyton, his writing and characterisation has dated in ways that the books of Rosemary Sutcliff, for example, or Stevenson, rarely do. The magic wears off.
Educators and critics may debate the virtues or shortcomings of CS Lewis all they like. He made one critical statement which has never dated and its wisdom resounds through the years:
"You can never get a cup of tea large enough or a book long enough to suit me."