I've read the Silent Planet Trilogy.
Out of the silent planet was a wonderful work of science fiction. It was rather realistic compared to a lot of Buck Rogers type settings. I really like the fact that C. S. Lewis had an alien society that was friendly rather than hostile, yet still had the mechanics and intrigue to drive an engaging narrative.
Perelandra was an engaging read in a similar vein. It has a good creative concept of paradise, and I like the non-linear texture to the world and the creatures and morality that other authors lack (Normally it's only the bad guys/demons that are interesting.) I did find some aspects of it philosophically off-putting and disturbing, particularly the protagonist having to murder a debate opponent for the good of perelandra rather than using reasoned argument. The lengthy discourse with the oyarsans got a bit tiresome.
That Hideous Strength differed from the previous two stories in that it is, wholly and primarily, a university morality play. The more you agree with the philosophies and counter-philosophies it presents, the more you will like it, and the more you disagree with it, the more it will cause you frustration and pain. The humor is well used, I particularly liked the detached-from-body old man character, and the objectivist became (perhaps unintentionally) laughable, but there are more than a few major points that philosophically rub me the wrong way, because they are sexist or just plain morally incorrect (The female lead should not defer to her husband in the matter of joining up the good guys because her husband is, frankly, a bit of a dope). The masculinity femininity thing isn't something I'm morally opposed to, but I just didn't see much evidence for or interest in that element. Lastly, after summoning all the oyarsans of our solar system, the actual means of destroying NICE seemed a bit anti-climactic.
All Merlin did was set the curse of babel and set the animals loose, and the whole raining fire and earthquakes was more or less redundant or clean-up crew work )
Throughout all of these works, I love C. S. Lewis's humor, his cosmic perspective, and his ability to filter the story events through the lense of an everyman so that even interludes with extraterrestrial archangels have the ring of realism.