And yet, of the half dozen or so times I have played it, the only good CoC game I have played pretty much ignored the insanity rules.
They worked pretty well for me for about 20 years or so.
I know some people like it but I definitely don't and I can tell you precisely what I don't like about it.
As a player in a game I have exactly 1 character I control and I hate losing control of that character for any reason. If I can't even dictate the actions of my 1 character, I may as well leave the table and watch a movie or something. If I am not going to be able to control anything that goes on in the story I may as well do it through a more visually stimulating medium.*
Now, it is inevitable that it will happen from time to time. I'll occasionally get knocked unconscious in a fight, fail a saving throw against a charm effect, get grappled by someone I couldn't possibly beat in a grapple, etc. I get that. But all of those are pretty temporary, lasting a few rounds or maybe an evening at worst.
But CoC has a core system that pervades the entirety of play that basically limits what actions I can take. What's worse is it seems to punish players for taking the most interesting actions. If you hear something scratching in a closet, your best bet is to walk away and forget you ever heard anything...unless you like going insane for some reason. The system doesn't say "Wow, you did something interesting. Here, have an Action Die." Instead it says "Huh, that was dumb. Looks like your f***** now."
The unfortunate thing is, I really do like the setting. I loved Dark Corners of the Earth and I thought their depiction of insanity was well done. It was interesting without being debilitating.
To be fair, I have only played CoC at conventions and convention games are notorious for their lethality. Most of those games, I wasn't a investigator, since I couldn't really investigate anything, but rather I was a Supernatural Psychic Punching Bag(TM). The one game where the sanity rules was ignored was the one game we really got a chance to explore and investigate what was happening, the one time the game had a payoff other than, "Well, looks like you are all bat-shit insane now", the one time the story was creepifying not because the game system said it was creepifying, but because we had a chance to actually find out what was actually out there to be creepified about.
* Note: I had a similar problem with OWoD Dominate. I would not play a character with less than 10 Willpower in that game.