I've had an iron warriors army for years. I've had an inquisitor warband that's been collecting dust for too long, a grey knight army, and a deathwatch army. So while yes, I'm already playing consistently, let me just say that these books are fantastic. They let me do whatever I want while building an army purely to fluff, rather than for power.
The inquisition book is awesome because it lets you take my 3 favorite grey knight units (dreadknights, terminators, and interceptors), deathwatch core units and their flyer, a master of the fleet (alone, which means I can drop my small IG army,) techseer enginepriests alone, sisters of battle, and it finally gives a physical book for inquisition.
Also I'd like to point out that the legions of chaos book doesn't force you to take formations. If for example I take a warsmith and 2 units of chaos marines, I can just call them Iron warriors (as long as I don't take marks) at which point I get free veteran of the long war and a 6+ fnp. The books are great for giving you options to do whatever you want without forcing you to do anything in return.