Back in the early 70s, I encountered this wonderful series through the first of the books to be written, The Lion, the Witch, and the Wardrobe. (Below, I quote most of my review of that book, insofar as it applies to the whole series). I subsequently discovered the whole series, and in the 90s read it to my wife, who loved it as much as I do. We didn't read it in this omnibus edition, but as individual books; and for a long time, I intended to eventually review each book separately. But since the series has so much commonality, I decided that reviewing it as a single entity is more practical.
Note: This omnibus volume lists the seven books of the series in their internal chronological order, starting with The Magician's Nephew, which describes Aslan's creation of Narnia; and this is the order in which Lewis himself recommended that they be read. Barb and I, however, read and experienced the series in the order in which the books were written. Lewis fans debate which order is preferable, and I can see both sides of that. Usually, my preference is to read a series in internal chronological order. But the way that we read this one probably provides for more of a feeling of resonance in the later ones, as certain things that were mysterious before fall into place later.
Most people know that C. S. Lewis was an effective Christian nonfiction apologist, using the tools of reason and logic to build the philosophical case for Christian faith. But he ultimately became convinced that an even more effective apologetic is available through the "truth of art," the instinctive and emotional appeal that stories exert -- especially the kinds of stories that draw on the deep, mythical archetypes of fantasy to illuminate the real universe. The Chronicles of Narnia, his classic fantasy series, was the fruit of that discovery, set in Narnia, a magical land whose world lies in another universe, in which magic works and time moves differently than it does here, and in which Christ is incarnate as the great talking lion Aslan. The first book of the series presents one of the most powerful symbolic literary presentations of the Christian gospel ever written. Although the intended audience, in Lewis' mind, was children (and his various direct addresses to the readers as author presuppose this), there is nothing invidiously "juvenile" about the quality of the writing; it can be enthusiastically appreciated by anyone who loves tales of imagination and adventure, fantasy and wonder; and the truths here, like those in Jesus' parables, are simple enough to speak to children but profound enough to challenge adults.
The Christian message is an essential part of all of the books in the Narnia series. We all react to fiction based partly on how we feel about the message(s) it conveys, and that's appropriate. So readers whose view of Christianity, or of religion in general, is highly negative could hardly be expected to give the Narnia series unqualified praise. (The converse applies, of course, to books like the His Dark Materials series by Philip Pullman, who avowedly seeks to be the "anti-Lewis;" it isn't surprising that his work is less appreciated by readers who hold a very negative view of militant atheism.) That's a subjective assessment, and fair enough as such. Some other criticisms of Lewis' series, though, are intended to be more objective, and can be debated objectively. (This discussion might contain some "spoilers.")
One reviewer states that the series "has no real conflicts" because Aslan can exercise miraculous power to resolve them. But if this is so, then the theistic view of real life is that it has no real conflicts either, since God has miraculous power to resolve them. But no theists that I'm aware of view real life in that way, least of all Lewis, as his other writings indicate (and insights from all of his writings are valuable in interpreting the Narnia books, since his thought was highly unified). As his writings on miracles make clear, he believed that God can intervene in the natural order miraculously --but doesn't do so very often, because intervening on a wholesale basis would negate the predictability of natural law (and leave us unable to recognize a miracle when one did happen!) And, very importantly, God doesn't make people's choices for them; they exercise free will, which requires that their choices have meaningful consequences --good or bad. So in Narnia, as in the real world, Aslan doesn't intervene very often; and most readers observe quite a bit of conflict. Bad things happen, and they aren't always deserved; evil isn't automatically and instantly punished; and good characters suffer and inevitably die, some well before their time. And characters experience a good deal of conflict in struggling to decide on the right course of action --or on whether or not to do what they think is right, when all the rewards would appear to gained by doing wrong. In one of the books, Eustace is indeed changed back from dragon to boy --but only after he learns a lesson about the value of human friendship; and that doesn't come easily to him. And in the first book, yes, Aslan will be resurrected after giving his life for Edmund --but his death is still an awful experience that he undergoes for someone whose welfare, viewed from a coldly objective standpoint, is nothing to him; most of us wouldn't undergo it, even with the guarantee of resurrection.
Like most non-vegetarians, Lewis views eating of meat as appropriate when the meat is that of a non- rational, nonthinking creature; eating a being who can speak is cannibalism, no matter what that being looks like. Whether or not one regards that as a significant distinction, or how significant it's seen as being, is a matter of opinion; but it is a genuine distinction between humans and, for instance, cattle.
Probably the most significant criticism here is the accusation of ethnocentrism and racism in the portrayal of the Calormen. Calormen are darker in color than Narnians; their culture differs from the Narnian one; and their government is a despotic empire that would like to add Narnia to its domains. (Neither Narnian nor Calormen culture are identical with any culture in our world, though like all fantasy writers Lewis uses this world's cultures as a grab-bag from which he can pull various features. Calormen is mostly desert, but its polity is much more Turkish than "Arab-like," and the idolatrous cult of Tash doesn't resemble Islam.) Some readers assume that any mention of dark skin means that the people so depicted have to be racially inferior; that race and culture are the same thing, with the former dictating the features of the latter, and that the character of a government mirrors the character of a people; and that if Narnia and Calormen's governments tend to be hostile and suspicious toward each other, that must mean that everything Narnian is good and everything Calormen is evil. But there are good reasons to think that Lewis didn't share these assumptions, nor want to convey them.
Two of the most sympathetic and positively treated characters in the series are the Calormenes Aravis and Emeth. Aravis is a strong, gutsy and capable heroine; she winds up marrying Prince Cor, and their son grows up to be Archenland's greatest king. And Emeth (whose name, not coincidentally, is the word for "truth" in Hebrew) is readily welcomed by Aslan into heaven, having amply demonstrated his moral worth. This certainly suggests that Lewis judges, and wants his readers to judge, Calormenes "not by the color of their skin, but by the content of their character." It's also instructive that a character in That Hideous Strength, Lord Feverstone, advocates "liquidation of the backward races" --but he's a spokesman for the anything-but-nice N.I.C.E., whose social program represents everything Lewis detested.
In the latter novel, closer to the end, Lewis lays out a theory of human cultures in which all of them, at their best and truest, are unique and distinct embodiments of moral and social truth, making a kind of truly multicultural mosaic in which the differences are respected and appreciated. This idea is reflected in The Last Battle, where Aslan's true country is made up of the Platonic ideal of every created country --including Calormen, where Lucy sees the towers of the true Tashbaan. So Calormen's cultural differences from Narnia can be viewed in this light --there is no reason to think Lewis' view of "shoes turned up at the toe, scimitars, suffixed phrases of praise, 'son-of' lineage declarations" was "unfavorable." The latter are found in the Bible (a book Lewis certainly viewed favorably!), and some of his writings suggest that he rather liked stately formal courtesy in social interactions. He contrasts the Calormen oral story-telling tradition favorably with English teaching practices; and if Calormen culture is called "cruel" in one place (which, Lewis would say, is a deformation caused by sin), it's also called "wise." Finally, King Miraz and his gang --who are all white-- aren't viewed as any more benevolent than the Calormen Tisroc and his toadies; the actions of both are due, not to race and nationality, but to the common experience of human fallenness.
This is far and away one of my favorite fantasy series. I'd highly recommend it for any readers who appreciate imaginative literature, and I believe most would find it both intensely entertaining and thought-provoking.