I'm not overtly familiar with the Cthulhu mythos, but I have read a few of the stories, and this game has an almost fanboyish love of the source material, and the atmosphere cannot be beaten. Places like Arkham Asylum (nee Sanitarium) are recognisable (and yes, Batman took the name from Cthulhu, not the other way around), and the section of the game I'm currently playing is in Innsmouth. Indeed, many of the characters you meet are from the short story, and being as I currently keep getting killed when fleeing from the hotel, it seems like this section is based on the story too.
Everything about this game screams quality at the top of it's lungs. Well, everything apart from the voice acting, but I'll let that pass. Graphics are rich and detailed, and the environments surprisingly large and complex. Incidental touches are suberb. Insects crawl around floors and walls. In-bred locals walk down dark alleys to relive themselves (something I couldn't believe when I first encountered it!) and the visual effects everytime your sanity is shaken are wonderful. But it's not just graphically superb, the sound design (apart from the voice acting) is equally as detailed. Even now, the sounds of a dead woman swinging slowly on her noose send a shiver down my spine.
According to the game stats, I've done about 20% of the game, and I've not yet picked up a gun or other weapon of any other type. But its not an action game. It's a game that's just about atmosphere and mood. In a way, it's a shame I have to go bolting doors, and jumping over rooftops now, as the somewhat arcadey/platformy gameplay is at odds with the rest of the game so far.
Now, if I could just escape the mob at the hotel, and leave Innsmouth... who knows what other secrets this game offers...
But I cannot recommend this game enough. It is superb. I'm also interested in some books about the expanded Cthulhu mythos if anyone has any suggestions for me.