So, I've had a once-through on the Khorne Daemonkin book. Some initial thoughts:
The book itself is well put together, with lots of nice photos and pieces of artwork, some recycled but some new. Still in the ork-organized 7th ed horrid layout. Army special rules on page 110, after all the units? Really now. The flipping required building a list is insane. I plan to copy some of the detachment and cost pages to avoid the madness.
As the name implies, this is a book about Khorne, and includes not only chaos marines but daemons as well, all Khorne-flavored. Everything is either a daemon of Khorne or has the Mark of Khorne built-in or is something that neither would apply to (for example a Rhino). If you're looking for a fairly interesting way to mix in Khorne CSM and Khorne Daemons, this may be for you.
As with many things, it may help to clarify what it is not. It is not Codex: World Eaters. It is not Codex: CSM and Codex: Chaos Daemons plus. There are very real restrictions, and some things left out that are kind of surprising.
You will not find a Chaos Boon table. Your champions don't get individually better through challenges, though they can help the army as a whole through generating more Blood Tithe points. You will not find Veterans of the Long War. You will not find Kharn. You will not find some simple vehicle upgrades that you would think would be Khorne-related, like Destroyer Blades or very useful for a Khorne army like Dirge Casters. You also won't find a Warp Storm table or Daemonic Instability (instead Daemons have just regular Fearless).
With everything having Khorne baked-in there are some advantages. Daemon engines don't just have the Daemon special rule, instead they have Daemon of Khorne so get Furious Charge and Hatred Daemons of Slaanesh too. Possessed appropriately enough get BOTH Mark of Khorne and Daemon of Khorne, so get benefits for both (at 30pts a model).
The blood-pumping heart of the book is the Blood Tithe. Essentially when a unit with the Blood for the Blood God! is destroyed or destroys an enemy unit, or character is killed/kills in a challenge you generate a point. You can trade these in during the game to summon units or give army-wide boosts to units with the BftBG rule. I am happy to see a non-psyker way for Khorne armies to summon in new units, especially as a result of what they do on the tabletop. At first glance this system would seem to encourage MSU type builds. Oh, and the result letting you turn a guy into a Daemon Prince lets you keep Warlord Traits if it was your warlord, and specifies if the DP model has wings, it will count as having Daemonic Flight. So, just in case you hadn't been doing it anyways, model them with wings.
The special force-org, the Blood Host Detachment, is a collection of formations from the book. Hence a lot of the page flipping if you want to use this. Look at detachment on p. 68. Flip past all the units to Slaughtercult formation on p. 103. Decide on Chaos Lord. Flip back to p. 72. Turn page over back to p. 71 for cost of his options. Flip to p. 118 to be reminded what The Brazen Rune does. Back to p. 71 to see costs again now that you know what it does. Etc.
A number of the formations have cool rules, but you will need to be careful playing as or against this army that rules don't spill over where they shouldn't. For example, the Slaughtercult formation gives you a bonus that your Blood Tithe is more efficient, granting a second, less expensive bonus for free...to every unit in that formation. Expect plenty of people to forget, and some to "forget" that that second bonus doesn't apply to their Brazen Onslaught, Gorepack, etc. formations.
That Blood Host Detachment is nice, especially as it generates an extra Blood Tithe point a turn and you could potentially really focus on a particular aspect that a normal Combined Arms FOC would not allow due to slot restrictions, but those formations add up fast. At 2000pts you're probably looking at a smallish Slaughtercult plus 1-2 other smallish formations or one formation and a few War Engines (ie Helbrute, Defiler, etc). For smaller games you may be better off with a regular Combined Arms FOC to allow some variety.
As one would expect, this is definitely an army about getting up close and beating face. Unfortunately, this edition of the game is not about that. While you can take some potshots depending on unit choice, pretty much write off anti-air. You have some anti-psyker stuff (bonuses to Deny the Witch and artifact that makes them more likely to perils for one turn) but with only 1d6 of dice to use you're not going to make a dent in a serious psyker army's ability to cast.
Overall, interesting book, one that I look forward to seeing on tables and trying out myself. There's certainly still room for the regular Codex: Chaos Space Marines as well, so if you haven't thrown it away by now, keep hanging on to it.