This is the fourth iteration of GURPS Space. It's got a lot that's new, quite a bit that's appeared elsewhere in more primitive forms, and a few relics that have been recycled from GURPS Space 1, 2 & 3.
Bad bits first:
The book recycles a small amount of faily bland and pointless text about interstellar societies and "generic" organisations in chapter 1 and expands on most of this information in chapter 7. Thus, some of this text is a bit redundant. It also includes the irritating joke organisation "Special Justice Group" (SJG, like the publisher, Steve Jackson Games).
There are no game statistics for equipment in the book at all, and no ship design system.
Steve Jackson Games uses the avoirdupois system in preference to metric. So do most Americans, of course, but it just seems unscientific for a scientific game book.
Good Bits
The vast majority of the book is devoted to hard science concerns; possibile and miraculous technologies, world and starsystem generation, and alien life. These are excellent. The technology section contains no examples, but is nevertheless thought-provoking and useful. The worlds section is as accurate and up to date as the authors could make it, and is a vast improvement on all the prior GURPS spaces and, for that matter on any star generation system by anyone else. The same goes for the alien life chapter, which is more detailed than any other that I've seen.
The remainder of the book has solid advice on how to create, fine tune and run a space campaign, and sample templates for character creation. It's all good stuff.
Finally, should you buy this book? If all you want to know is "how does a 3rd Ed. TL8 gauss needler translate into a 4th Ed. weapon?" then no, because the book doesn't tell you that. But if you want to design and run a scientifically plausible, richly detailed space game for any games system, then yes, this book is definitely worth the money.