Like Mike, I also count myself a fan of Lovecraft's work. Being quite familiar with his writing, it seemed to me that bringing a faithful interpretation of his stories to life as a videogame was no small task. However, Headfirst has clearly done their homework and it shows.
More than anything else, Call of Cthulhu: Dark Corners of the Earth truly captures the dark, oppressive and even hopeless feel that many of Lovecraft's stories have in common. Walking through the town of Innsmouth in Jack Walters's shoes was exactly the way I had imagined it to be when I first read the source material, and as Mike says, the attention to detail and small touches of realism help embellish the experience and make it a (mostly) believable, immersive adventure.
Without a doubt, the first third of the game was my favorite. Finding clues and acting like a detective got the nightmare off on the right foot, starting slow and building tension before the true horrors in the town were revealed. Once things went to hell, being forced to explore and evade the misshapen inhabitants without a weapon was intense and stressful, exactly what it should feel like if an entire town turned against you. Dark Corners of the Earth's opening gambit feels like a very conscious re-imagining of what a first-person game can be, and I appreciated their efforts immensely.
The rest of the game is good and solid, although it doesn't always manage to sidestep common PC pitfalls like giving too little guidance through idiosyncratic puzzles or expecting me to make unintuitive choices that aren't logical or in tune with a coherent game world. For example, I couldn't understand why I kept getting killed by dynamite when I was running for cover a hundred feet away and then ducking behind a thick stone wall. I eventually discovered that the developers wanted me to hide in a special hole five feet away, I simply hadn't tried it because it seemed too close to be safe. Another time, I kept getting "washed overboard" by a tidal wave even though I was inside a ship's cabin behind a locked door. That time, I was supposed to grab onto a railing that I didn't even realize was outside on deck. (And the final battle? It's not hard, but an FAQ is required.)
Call of Cthulhu: Dark Corners of the Earth also bogs down in the middle with some item-fetch situations that don't really add much to the overall story except to make it longer, but despite the bits of fat and some quirky solutions to odd situations, the intriguing story and Gothic horror atmosphere override all rough edges to create a unique experience incorporating both electronic and literary influence. I agree with Mike when he says that it's not necessary to be a fan of Lovecraft's tales to appreciate Dark Corners of the Earth, although those who are will be able to fully appreciate how completely Headfirst has captured their subject. For newcomers to the horror, strap yourselves in and then make a trip down to a local library or used bookstore after the credits roll.