The Role-AIds line was a series of supplements, modules and even campaign settings writing with the AD&D rules in mind My box says "Presented by the editors of Role Aids for use with Advanced Dungeons & Dragons role-playing game." Some of the early Role-Aids stuff was officially licenses, but apparently that relationship had ended by the time that this line of supplements came out.
I've used swipes from the Demons stuff in my online Swords & Wizardry game. There is a lot of interesting material in this boxed set (and all of the others as well): demonic monsters, delightfully twisted new spells (admittedly a lot of which have to do with summoning and unleashing the nasty new creatures to be found in this material) there is even a new class and a new race. They are even nice enough to describe a cosmology into which these new revelations fit (as well as future ones in the other supplements). All of this in about 100 pages, or so. Succinct.
The material in Demons is courtesy the imagination of Mike Nystul. Yes, the Mike Nystul who has a D&D spell named after his real name rather than a character. Also, the same guy who created Whispering Vault and did a bunch of stuff for Mayfair's other game lines, and worked on Shadowrun as well. That gives Demons quite a pedigree.
If you hunt around eBay or places like Noble Knights or Troll and Toad, you can find copies of stuff from the Demons line for fairly decent prices. I do need to track down a copy of the Apocalypse boxed set, I think that is the last thing that I need for this line (one of the very few game lines that I am kind of completionist about).
I think that Demons is ripe for someone to make an "homage" to the material. I would probably do it myself if I had a good enough of an artist that I could afford to pay out of pocket. There's nothing more old school than having a bunch of demons and demon hunters to stomp all over them. Just writing about it makes me want to run that sort of campaign. Needless to say, I really enjoy these supplements and think that you should check them out as well, if you haven't already. There is some really cool stuff that can perk up your campaigns to be found in all of these books. Not too many random tables, but a pack of interesting monsters, spells and a demon slaying class can be found for you in this box. Not to mention a new look at a universe that you've been gaming in, but that still has some new secrets for you.
Oh, and a slight disclosure. When I was still with Seraphim Guard I did spearhead an attempt to purchase the rights to the Role-Aid line from Mayfair Games. I thought the original material would be of use to people still gaming with the old systems, while at the same time brand new material could be spun out of all of these great books for people and their games now. Sadly, it turned out that Mayfair no longer had the rights to this line. Apparently in their legal battle with TSR over the copyright/trademark issues over the Role-Aids line, the rights had been transferred to TSR. That means, someplace in a WotC archive all of these lovely Role-AIds supplements are mouldering away.