FINALLY! This is the book I've been waiting for. I love Rogue Trader, but don't have the time to generate my own Xenos nor do I want to hand-wave too much in order to preserve balance. This book provides me with all the nasties and big bads I need to punish, er, challenge my players. It's 146 pages of pure sadistic delight. It's organized pretty easily. There are beasts, sentient aliens, creatures of chaos, and your very own quick generator for dangerous plant, animal, and Xenos life. As with all the books, the art is off the hook and really shows the brutality and ferocity of these foes.
You probably haven't seen a lot of the beasts in the source books so far, so I'm digging the new options. It's always fun to throw some feral creatures at your players. I like the diversity. There are great narratives about various Traders running across these, giving you plenty of idea of how to ambush unsuspecting PCs. There are a pretty broad range of just animals to formerly sentient beings and a good selection of environments you might find the things.
With the aliens, I'm glad to finally see a lot more Eldar. I don't play the tabletop game, so I don't have stats at the ready and these are some of the most dangerous Xenos you can pull out of your pocket. All the Xenos have background on their race, motivations, etc. The Orks have a good number of options (you can make those as PCs though also). Then there are a handful of the less well known species. The Tau and some of the previously described races are left out here, but I'm fine with that.
The creatures from the Warp are sure to be the biggest challenge to your players. These things corrupt by just being near them and have huge ramifications on the characters' psyches. I'd probably use these sparingly unless you are unusually cruel. The base stats may not always seem all too crazy. I'd suggest you look at the traits, which more than make up for a 70 Weapon Skill. Again, great representations of the creatures and descriptions of just how evil they really are.
If you plan on running Rogue Trader, and I plan on running it a lot more now, this is a must have. Whenever you feel like it's time to add a hurdle, pull out something nasty and see how many limbs it can remove before the PCs can take it down. I'm so happy right now.