As said many times in various reviews for this book, what is in the book is excellent. What is not in the book is what I want more of.
I appreciate the storytelling, the various "files" on the xenos & other creatures in the book... but for the most part, "Creatures Anathema" seems like half of a great book.
I was particularly interested in the Eldar & Tyranid entries in the book. Whilst I can get by with the Genestealer & Lictor entries for the Tyranids, the authors simply waved away Genestealer hybrids by saying that they are outside the scope of the book. So you either have to make up your own rules for hybrids & infestations, or the Genestealers are pretty much a non-factor for you.
Likewise, the Eldar section begins with a beautiful picture of a Warlock... but no Warlock stats, no Eldar psyker abilities, and not even the stats for a standard Shuriken Catapult (ostensibly because there are no rules/stats for your standard Eldar Guardians). I can certainly make my own stats for Eldar Guardians, and I appreciate the usefulness of Eldar Pathfinders & the fact that Dire Avengers will be more commonly encountered than Warp Spiders or Shining Spears... but why no Eldar psykers? It's a glaring omission, especially since we get psyker abilities for summoning rats in this book. (Oh, yes, you can play as an unsanctioned rat summoner now, thank the Emperor, but you will not find rules for Eldar Warlocks) I guess I can also create my own Warlock abilities, and say that Farseers (like Inquisitors) are far beyond the scope of Dark Heresy... but there's hardly enough meat to put together a suitable Eldar encounter without knowledge of Eldar from the tabletop miniatures game.
Instead of more details on Genestealer Hybrids & Eldar wytches, you get more Calixis sector stuff. While several pages of non-minatures game beasties are appreciated, you get 34+ pages of non-sentient vermin/beasts versus 6 pages for Eldar and 8 pages for Tyranids?
Granted, the Eldar could have an entire book to themselves... but maybe they deserved more pages when you have so many beasts/vermin entries? Maybe an entry for Guardians or Eldar corsairs (since you shouldn't have to buy Rogue Trader if you don't want to)?
That's the frustrating part of the book. There's some great stuff, but it requires a lot more work if you are interested in Xenos threats.
For Ordo Hereticus games, don't even bother with this book. It's interesting, but not worth your time IMO. Get the other books in Dark Heresy, or get the Rogue Trader rulebook instead... you'll get info on Eldar Corsairs & Ork Freebooters as well as Kroot mercs, in addition to Rogue Traders & Astropath characters, which is FAR FAR faaaaar more useful than what you'll find in Creatures Anathema.
Rogue Trader RPG: Core Rulebook (Warhammer 40,000 Roleplay)
For Ordo Xenos or Ordo Malleus games, you'll find useful info here (moreso for Ordo Malleus, with Flesh Hounds & Juggernauts of Khorne, plus Flamers of whatshisname)... but Ordo Xenos acolytes in particular will need more help from a creative gamemaster to round out what's in this one.