He'd murder my PC Dark Apostle without effort despite having a much inferior version of the Crozius.

Remember that even a starting chaos psyker can give a Warp Time bonus of 6-10 iirc. NPC chaos characters don't have the benefit of a friendly psyker buffing the hell out of them. Not to mention that PCs have the benefits of infamy, and all sorts of items and stuff to boost them up.

I want to focus on the average citizens and militia that turn from the emperor rather then the usual chaos space marines and daemons.

Only War core has plenty of stats for chaotic humans. You can combine those with the Marks of Chaos in this book to have Khorne Rebel Guardsmen or what not and the formations definitely work with these.

From what I saw, the Marines listed here are actually significantly stronger than the general ones in Deathwatch. They're still beatable, though. Nail them with a lascannon, flood them with Krak grenades to stun them and knock them over with Concussive (admittedly, the Marines my squad successfully used that tactic on weren't as powerful as the ones listed in the book), use Autocannons on them because Autocannons kill everything. It'll be a hell of a rough fight to take one down as Guard but like a lot of the bigger enemies in the new book, you have to take advantage of heavy and special weapons.

I actually found the book most useful for actually fleshing out Severus as a villain. He's actually quite interesting to me, being a greedy jackass who really isn't corrupted by Chaos (yet) and is actually acting on human reasons. I also like that he's actually slowly losing the war and isn't secretly some super power murder machine; he's actually a pretty sweet villain now and makes me regret slightly that I'm not running the Spinward Front. The fluff stuff in Enemies of the Imperium, combined with the formations, and having a few more nasty foes to throw at a squad, makes it pretty worth it to me.

Ok so I got the book and finished reading through it, here is what I think.

Severan Dominate- I really like these guys, the way their society works is very interesting and on the surface seemingly better then the imperium and a more appealing form of government. I really like how it says on their free time the soldiers are put to work doing the jobs of civilians to aid in the war effort, in effect allowing them to make the equipment that will be used in the war they are fighting. I also Like how rules to create severan dominate characters were put into the book.

Orks- while the nobz, meks, and warbosses can already be found in the final testimate books the other entries in this section really give some interesting challenges to players as well as giving a challenging fight to baneblade crews.

Kroot- An interesting (if somewhat unusual) choice to include a mercenary force. While I doubt they would be used alongside imperial forces like the book suggests (since they are still xenos) I can think of many fun scenarios in which to use them.

Dark Eldar- Finally they include the Dark eldar units people want to face. Archons, succubi, haemonculi, incubi, etc. This will really help GMs create a campaign that focuses more in depth on the dark eldar rather then the basic troops they show in the core rulebook

Chaos- Well we all knew this was coming, the hellbrute and infamous chaos space marine give players a chance to face down enemies that will terrify them and almost each of these entries in this section will make a good 'boss' for the players to face, I wouldn't send more then one of these at players at a time unless they were packing some serious firepower.

Formations- Though it makes squad level combat easier to handle for GMs I was disappointed because I was under the impression that it would help GMs manage larger battles (such as company or regiment strength battles), instead it introduces a similar feature to the game that hordes did. Sadly it only aids in squad level combat and while useful and well done does not help manage those massive battles that the Imperial guard is known for participating in.

Well, having just fought a Chaos Marine and squad of renegades using these stats as a 2500 EXP Heavy Gunner, I can say I can see why they made them tougher. I kinda wiped the floor with the Marine via autocannon while my squad set the renegades on fire and grenaded them. I don't think we even took a single wound and we weren't especially high level or powerful. Admittedly, that took a fair amount of good luck (Marine failing Dodge, good burst with an autocannon, a bolter jam for the Marine) and if you take a single hit from one of those buggers you're pretty much done, but the prevalence of heavy and special weapons in a Guard squad really does make them a ton more manageable than you'd fear, even with those crazy stats. Was still a hell of an exciting fight, though, which means mission accomplished. After all, had we not gotten so lucky with our shooting that thing could've hit melee and we'd have all been screwed.

The Order of St. Meleum is a nice bonus, too.

Picked up this due to this thread and while I haven't thrown anything in power armour at them again I'm LOVING the formation rules. Granted it has given pause to the commissar...correction it has busted the commissar up but being a religious fanatic he keeps charging and I can't help but love that. The ogryn is giving balance issues BUT formations have actually made lasguns pretty good now. They can kill off people relatively easily as long as the shot doesn't hit cover.

The formation rules are a great addition to speed up combat. We tested them and loved them...