I had the opportunity to review 'Rites of Battle' recently and concluded that no 'Deathwatch' GM could run the game without it. Given the focus of 'Rogue Trader', I feel exactly the same way about this book. Chock-full of information which simply would not have fit in the main rulebook (a hefty tome in itself), this definitely required a separate supplement.
Perhaps the authors wanted to start with a bang, as it jumps straight into a vast array of new weapons which are simply devastating. Torpedoes are given a full treatment, followed by attack craft, then the impressive Nova Cannon and rounding off the chapter with a selection of new starship hulls. Each hull comes with an illustration in the best 40K style - aficionados of the wargame will not be disappointed. In fact, with 11 pages dedicated to new starships, the players and GM alike will not lack for choice. Having played a number of sci-fi RPGs, I can say that I'm always on the lookout for new vehicles, and this sentiment is obviously shared by the games' designers.
Chapter 2 gives an interesting overview of the form and function of the Imperial Navy and provides a range of interesting nuggets of information which can be seamlessly woven into your campaign. Chapter 4 likewise gives a host of new information for the game in terms of space warfare from squadrons to scaling space battles and is well worth the read.
Chapter 3 was actually the first one I read in the book. As a GM for 'Dark Heresy' and 'Deathwatch' I'm on immediate lookout for anything Xenos, and this chapter, whilst designed for 'Rogue Trader' is eminently useful for the other game lines too. Herein you'll find Orks and Eldar (among others) with new weapons, information and of course ships. In my opinion, the most useful 30 pages in the book, although I shan't disparage the quality of the other chapters.
The book has internal links which I have come to expect from FFG e-publications and this improves navigation immensely, especially as I see this primarily as a reference book. As I mentioned before, the artwork is wonderfully faithful to the wargame and the layout logical and readable.
I can't honestly see how one runs 'Rogue Trader' without a copy of this book and 'Into the Storm' as they form a core trilogy of purchases for any newcomer to the game.