I gave it three stars, but this book is not mediocre. I had to give it that because of the huge rift created by the awesome imagery and storytelling - some of the best in the series - and the judgement Lewis thought it was necessary to heap on certain aspects of modern society. I mean all the sexism, and the comments about Eustace and Jill's school, Experiment House, and its liberal attitude, lack of Bibles, gender integration, and stance against corporal punishment. There's a ton of this in the beginning of the book, and then, like a drunk uncle at a child's birthday party, it periodically pops up to crap all over what's happening.
If you cut out that garbage, you have a great story about two schoolkids on a journey to hell and back to search for a lost prince. I really liked the character of Puddleglum, the cheerfully pessimistic Marsh-wiggle who accompanies them. And everything that happens while they are in the underground city was amazing.
Read it, but try to skip over the crap if you can.