I have read them as an adult, while reading to my kids. Yes, they are Christian propaganda, in the technical sense of the word, not the post-Nazi sense. They were intended to tell a Christian story and they do, most obviously with Aslan’s resurrection. So what? Are the only interesting children stories non-Christian? Can not a nonchristian learn something of value from a Christian set of stories?
When I started writing this, I thought the misogyny claim might be a better founded criticism. The villians in two of the books are women (the White and Green witches). But wait, there are more than four books, so that means he’s misogynist for not having enough women villians! (Well, the White witch appears in two books, but the uncle is more the villian in the one that explains the origin of the White witch.)
To knock him for Susan is a bit silly. The books are intended to tell a Christian message that children can see. Someone has to reject Narnia and take the role of the unbeliever. Lucy is the hero of the series, so she couldn’t be “the one” to reject Narnia. (In the second book, only Lucy can see Aslan at first, and Lucy is clearly Lewis’ niece for whom he wrote the stories.) Edmund had already done his bit as the bad guy, and the High King Peter couldn’t or Aslan would not be all knowing. Plus, I assume he was given the name Peter for a reason. Jill and the cousin, whose name escapes me, had already learned their lessons. That leaves Susan to be the one more interested in worldly things than important (to Christians) things. If you think about it, two of the three boys (from our world) are bad at the start, and all the girls are good, although Jill is not perfect.
I would criticize him for the Calormenes. The Narnians are clearly northern Europeans, and really, English. I suppose the big rival kingdom could be French, or something, but it is not unreasonable for him to have modeled them after the Ottoman Turks, who were Europe’s only recent, outside military threat. Yeah, he’s got the Telemarines, or whatever they were called, and they are European, but the Calormenes do seem to be stereotypes.
As stories go, I originally thought they were too simple, when I read them as a kid. I like them better now, which is odd. My kids love them. I think because they like the people and characters. They also love Harry Potter. I think more because of the clever inventions. To call either set loveless is crap.