If you're considering playing any kind of grim, gritty, realistic, macabre, horror, or generally cthullhu esque game, I HIGHLY recommend giving these a try!
A character can only see so many horribly murders, face down so many terrors from the abyss beyond, or suffer SO much psychic damage before it starts to leave an effect on that character's mind, and these rules are a fantastic way to consider implementing that. From becoming panic prone - which gives a benefit to initiative at the cost of a penalty to certain saves - to being sickened or paralyzed with fear, or terrified of an ill omen, or, or, or!
There's a lot to add to your gameplay in here, starting wit the addition of sanity effects to many existing spells, which opens up the possibility of either driving your players OR your enemies insane through magic. A few fitting sanity rules for certain classes and a couple of sanity oriented feats round it all off make it a great add to ANY campaign, but especially so the ones I listed above. Personally, I'm going to be using a lot of this stuff moving forward.
Oh, and best part? There's a one page cheat sheet at the end for easy use as a DM.