Well... I have the Demons 2 boxed set. Overall, it is very nice both in presentation and content. [Takes ominous black box off the shelf. Spooky music.] I will give you a step-by step tour. If you have further questions, just ask!
There is a pamphlet, which is the diary of a demon hunting inquisitor (which is pretty an entertaining intro into the methods of the "righteous"). And there is another one, written from the perspective of a demon-summoning order. Both of them would make excellent handouts - they have nice "illustrations and diagrams" and whatnot.
The meat of the box is the "Gamemaster's Book". Most of it is completely rules-independent, so you can use it in D&D, HM or even Ars Magica (more on this later...) It has a short introduction into the nature of demons, their favourite means of corruption (from "infidelity" to "false crusades"). There is a treatise on demonic gates (examples provided - again, there are many good ideas to steal, like "The Ruins of Ibn Suf", which is a famous "oracle" in my campaign).
We also get "signs and omens" - small signs that tell you a demon is nearby (soured milk, women miscarrying, etc). You can afflict your hapless NPCs, PCs and objects with demonic taints, or give them "gifts" (heh, gifts...

), for the appriopriate price (which is always
high ). A sampling of "demonic artifacts" are provided (not artifacts in the D&D sense, BTW, just really twisted magic items - from living (and malevolent) gardens to small worms that remove diseases (and... other things...) when ingested.
Demonic cults are detailed, from the nature of the leaders to their rites and, again, samples (like the typical witches coven or the "degenerate orgiastic society") and an adventure seed for each. The Cabal, the greatest of all cults, is given a whole chapter (only problem is that it may not fit to all worlds...).
"Soiled Seed" is about half-demons and their powers. This is standard fare, really...
And finally, we get the Inquisition, with its own spells...
The fourth manual in the box (did I mention that there are four booklets?) has two adventures - both focus on investigation, but I haven't read them yet, so...
Overall, it is a very good supplement.... with one small problem: it isn't an AD&D box. The outlined world isn't traditional D&D (more like Ars Magica - there is a powerful church, magic seems rare and the whole thing is very medieval), the stats are extremely sparse and you have to do a lot of work to actually
use it - but I would still buy it for the ideas!