I read this book to refresh myself after reading some long-winded classics.
By today's standards, the plot is not original. However, the books reads clear and easy, the dialogue and action are quick and direct, and you can get easily lost in the fast-paced story Heinlein lays out. If you want to read something quick and enjoyable, this is a decent choice.
Mary's character left me unsatisfied. Sam is obviously a typical character, but the way in which he interacted with Mary, and she succumbed to everything he asked in time, even if she put up a little resistance. If she is supposed to be such a tough agent, wouldn't she be capable of making decisions and asking Sam to follow them? Anyway, this book wasn't intended to be a character study. That was really my only qualm with the book