I enjoyed Prince Caspian (and, honestly, it takes effort on Lewis's part to write a book that I did not enjoy), even though I was expecting it to reach the level of its predecessor and it is never quite that good. That's not to say that it isn't a good book. It's just that The Lion, the Witch and the Wardrobe was better.
Still, for all that I'm faulting a book for being something it's not, Prince Caspian is yet another example of what sort of world Lewis can create and, seeing as it is Narnia, it's difficult not to love. Lewis also has not yet brought the allegory up to sledgehammer level, so the book can be read on the level of an excellent adventure story as well as as an allegory.