This book is fairly similar in format to the Enemies Within and Enemies Without, so if you are familiar with those two books then you know what to expect. There's a bunch of new worlds described with some interesting plot hooks, and some player stuff related to Ordo Malleus daemon hunters- special weapons and ammo, talents etc... It does include some new psychic powers, mostly related to demonology/summoning- I haven't looked at them too closely yet, but they seem like they would be of more use to villainous NPC psychers than PCs. That being said, an extreme radical acolyte might make use of them.
Of the three books I think that my favourite one is Enemies Without, but I like that there a lots of stat blocks for various daemons- not just Nurgle related ones (which was all we got in the Core book). I GM, so for me that is the highlight of this book, and my main reason for getting it. It should be noted that Daemons are pretty powerful RAW in 2e DH- for example the hellblades of the Bloodletters do something like 1d10+16 damage with a 5 Penetration- not sure how balanced that is. There's also rules for creating daemon princes, but I haven't had a chance to delve into those to deeply yet.
[4 of 5 Stars!]