Lewis has a gift for world building. I've climbed into a wardrobe or two in my time. Out of the Silent Planet's Malacandra isn't quite as vivid as Narnia, but it's close. The different alien races here are fascinating. My one great regret about the book is that it doesn't spend as much time as I would like getting to know them.
Late in the book, the three Earthmen are summoned to Oyarsa, Malacandra's invisible, divine (or angelic) ruler. The blustering Weston tries to take charge, and the result is way funnier than I had expected. It skewers first the "intimidate the natives by speaking very loudly" school of exploration and the sort of social darwinism that was in fashion at the time. Sadly, neither are too outdated that they aren't still worth skewering. Actually, all of the dialog is good, and the conversations Ransom has with other characters are the best parts of the book.
This is C.S. Lewis, so there is indeed a Christian allegory in here. It's far more straightforwardly so than the first few Narnia books had been, but I didn't find it intrusive.