Like all his work these are great reading, though very different from what a normal SF fan would expect. Partly due to the difference in worldview, and partly just because he was writing in the 1940's when ideas about other worlds had little scientific basis to go by.
I would particularly recommend That Hideous Strength, which in my opinion is a much better book than 1984, while covering the same ground. I'm not the only one who in recent years has expressed the thought that we are living out the world depicted in That Hideous Strength.
My favorite CS Lewis quote—most here probably have heard it:
“Of all tyrannies, a tyranny sincerely exercised for the good of its victims may be the most oppressive.
It would be better to live under robber barons than under omnipotent moral busybodies.
The robber baron’s cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience.”