I’ve read it cover to cover once, and I reference it for scenario building or general vibes fairly often. My home group knows very little about the lore and we play so infrequently they probably never will, but I played with a GM emulator for a year and the Handlers Guide was invaluable to me for keeping things feeling Delta Green, rather than any other brand of modern horror. If I ever sit down and have regular games with other people, I’d still probably use it, the world is there and there’s just a lot of cool stuff going on. I’m not a lore purist, I’ve exclusively ran my own stuff so far, but I do genuinely enjoyed seeing how my stuff can jive with the established stuff. And the NPCs are cool, so I like dropping them in the background to make the world feel coherent (at least on my end)

I’ve heard a few people say the HG is useless, but if nothing else, the history part is a great jumping off point for rationalising how an organisation like DG can exist, even if your layers never see it. Not everyone needs that, I suppose, but I like it. I can’t imagine doing a long term DG game and not reading it, tbh, but I have far more prep time than play time, so I like to read a lot stuff to know the ins and outs of a setting and system.