A great many reviewers here, and many other people who have talked about or reviewed this book, are disturbed or unsatisfied by the ending. However, the ending is what Heinlein was trying to do.
The adventure and romance that makes up the first part of the book is interesting and exciting. There are some very good action scenes, something Heinlein does well but not as often as some others. There are plenty of hints at sex. The characters grew on me. This isn't, as someone suggested, a parody of a fantasy/sword & sorcery novel. Heinlein is respectful of the material. The resemblance to his friend Poul Anderson's Three Hearts & Three Lions is deliberate. However, he is using the sword and sorcery novel and all its tropes to reach his central question, not just for its own sake.
The question the book asks is "what do you do after you've slain the dragon?" and that question is central to the book. I'm not saying that Oscar answers the question very well, even for himself. I'm sure that he doesn't answer it for every dragon-slayer who will come along. But Heinlein, opinionated as he often is, usually leaves us with questions unanswered and things we have to think about. Which is good.
I gave it only four stars because Heinlein wrote better novels than this one. But 4.5 would have been more appropriate.