This trilogy is one of my favorite works of sci-fi. Very moving stuff.
Out of the Silent Planet is a neat little pulp-style sci-fi story, with a slight Christian metaphysical angle, and a somewhat simplistic alien culture (i.e., the nature-connected hross, the coldly scientific sorns, and the industrious, artsy ummm… other creatures. Pfifltriggi? Can’t recall.
Then comes Perelandra, which, IMO, is an absolute powerhouse. Lewis’ ruminations on the nature of evil, the very good reasons why human beings have emotions like hate (to fight evil, with our fists as well as our hearts and minds), the terrible sacrifice of Christ, the divinity that an unfallen Adam and Eve must have possessed, and his brilliant apologia for the nagging unfairness of the biblical Eden story (that God was going to give Adam and Eve all of the knowledge that the trees’ fruit possessed, if only they would show that they could follow his instructions for a little while).
And this praise from an atheist, mind you. 
That Hideous Strength was a bit more tedious, in my mind, but not wholly unsatisfying. It was interesting to see him integrate Merlin into the Christian worldview (namely, by showing that he doesn’t quite fit, and that he bends the rules rather more than the powers of good might hope), and to show the banal, vacuous side of evil (in a university, naturally).
An interesting point he brings up throughout (which his compatriot Tolkien touched on in the Silmarillion) is that misdeeds have consequences, and though God can ultimately surmount any obstacles humans may lay in the way, it’s not without a price–often terrible–and the original state cannot be restored. Thus, the protagonists of THS were to have borne a child that would help save the world, etc., but they refused to heed such designs (pursuing “modern” notions of equality between the sexes, etc.), missing their chance, and leading to much trouble.