You gotta be careful when taking people out of D&D for the first time and dropping them in CoC. If it's for a one-shot, then it's perfect. Go for it! I'm a big CoC fan. Get the totally amazing Starter Kit box and have fun! However if it's for a campaign, you're facing 2 paradigm shifts:
The first one is learning a new rule system with so many new or different concepts... for example, for some obscure reason, many D&D gamers have a lot of trouble getting used to a "roll under" system because "roll high" is so ingrained into their brain as what TTRPGs are about. But on top of that you have separate skill scores (as opposed to a unique proficiency bonus), extra stats (SAN, etc.), meta currencies (if you're using Luck rules), etc... that's a lot of brain power used up for groking the system. In my experience it goes OK in the one-shot, but in a campaign, on session 3 or 4, some people start mentioning/complaining that they "still don't quite get it" (although in my experience they're still engaged and trying).
The second one is the one to be
most careful about. Horror gaming is IMHO unique among TTRPGs in the sense that it's usually the first game that actually changes the relation between player and character. In "traditional" RPGs, your character is something you care about, which becomes better with time, making you feel awesome and heroic. PC death is a big deal and a major event. In horror games however,
characters get worse with time. The predicaments they get into are supposed to make you afraid. Because of corruption mechanics like SAN, and the general trope of things being hopeless and grim, there's no way you can have the same power fantasy relationship you'd have in D&D. That's a vastly different way to look at the game. As with many many other things in RPGs, it's all about managing players' expectations, but this aspect of horror gaming is IMHO often overlooked in favour of "
let's check if the players are OK being scared with spiders and human sacrifice" (which is also important to check, mind you). And yes Pulp Cthulhu kind of addresses that but I haven't played with it yet.