If you expecting a book about the guys who created Linux or founded the open source movement then look elsewhere. What you get here is a discussion on ethics and the philosophy behind meaningful work which, to my mind, is much more interesting.
In particular, Himanen's comparison of the hacker ethic to the protestant work ethic struck me as apposite. There's lots of other good stuff in there too, including a great joke about God designing the earth by committee (well, it made me smile).
The introduction by Linus Torvalds is certainly worth a read, although I found the final chapter by Manuel Castells a little verbose - to me, it was stylistically quite different from either of the other authors and seemed out of place.
All in all though, I'd thoroughly recommend this book. It's a quick read and most people will get something out of it.