I first saw Mary Roach on an episode of the Daily Show, promoting some other, later book. I'd never heard of her, but my wife and I were immediately impressed, and agreed that we should pick up some of her books. Soon afterward, happily enough, I was given this book.
It's great. It fulfills its promise, and is just a lot of fun. Roach shows an unabashed willingness to dig into the crass, vulgar parts of space travel - the parts a five year old might ask about. She deals with all sorts of interesting practical questions that you generally don't hear about, and NASA might wish you still hadn't (how does NASA deal with penis size? Who knew they did?). She writes in an engaging, easy-going style, covering topics from what really happens when your suit leaks during a spacewalk to developments in low-gravity excretion. I'm pleased to say that she devotes a fair amount of time to the various animals used (pretty harshly) in space exploration, and if she's not as sympathetic as I'd have hoped, she does treat them with far more dignity than NASA did.
Roach is not afraid to go off on all sorts of fascinating tangents. That's one of the strengths, but also the fundamental weakness of the book. While it's titled "Packing for Mars", Mars itself plays a pretty peripheral role - largely because there's no real plan to go there. Mostly, though, there's no clear destination in the book itself. It covers all these great subjects, but it doesn't add up to anything in particular. Despite that, it's a very good book.
If you've wondered about some of the practical mechanics of space travel - not how they make the rockets, but how people actually manage - this is the book for you. Don't look for any particular purpose - treat it as a book you dip into now and then for a chapter or two, and you'll be delighted.