Damn, there's so much to like about this setting and system, but the more I read, the more all that was covered by stuff which I explicitly didn't like.
The mechanics here are roughly what Warhammer 2nd Edition was, but with a bunch of changes - some of them pretty good, others adding what I see as somewhat pointless complexity on top of a system that never felt like it needs to simulate very specific situations. There's a few pretty unique systems here - like the Subtlety track and Inquisitor Influence - but both seem mishandled, at least on paper. They feel like busywork which puts numbers on top of narrative stuff which should pretty much stay narrative.
For all the richness of the setting, the game feels severely limiting. All those different characters you could play feel like they can be boiled down to either different kinds of fighters, or a tech/science person, or a psyker which plays as either of the previous categories. For all the small modifiers, weirdly specific class "powers" and the plethora of gadgets available, it doesn't seem like the game allows for many playstyles.
It's definitely geared towards a very specific way of running scenarios, as the Game Master chapters seem to suggest by being very perscriptive. They're written in a way which says "You need to do this, you have to do it this way, that's the way you should structure play." way too often for my taste. If that's the intended way of playing Dark Heresy, then I'm not sure if it's inticing enough for me - there's a big focus on either stealth, or straight out combat, and non-combat abilities (outside of social skills, which seem essential) look to be useful, but simplified to just single tests to see if your character has in-universe knowledge.
Maybe I'm overthinking this. I'm hoping to run a game or two before my final verdict, but from the first read, the corebook is a bloated monster, with way to many specific rules for my taste and with a lot of lore and fluff which somehow doesn't make me feel any more informed about how I should run this game.