The pros: the atmosphere in the beginning of the game is excellent. Reading documents, seeing strange things, it all works very well.
The stealth game play works, for the most part. But make sure you enjoy FPS stealth, it's not for everyone.
Some good level design, throughout.
Overall excellent first half (although most of the bang lies in the first two chapters).
Some interesting game design choices that are rarely seen, such as the lack of a HUD and the healing mechanism, makes the game more immersive and seem fresh at first...
The cons: ... but exactly the same things become tiresome and irritating when the game becomes combat oriented.
Graphics are horrible for a game released in 2005. I mean, it looks barely better than Half-Life, released in 98!
The gun play is terrible. A few, boring and generic weapons you used to death in other FPS. Really awful and becomes preponderant in the second half of the game.
Boss battles... well, boss battles will probably seem ok for some, but for others (such as myself), they might seem like the worst part of the game, depending how much you understand of the Cthulhu Mythos. Sufficient to say, anyone who is familiar with it's concepts will feel cheated, as they are played out wrong (I'm being vague for spoilers).
Story development, quite interesting at first, becomes very dull in the second half, where the lack of documents is almost criminal...
The second half is just plain bad, all the tension ebbs out.
The 'twist' ending gives us something so dull and pointless that you're better off stopping beforehand.
Overall: This is a very unbalanced game, which starts with a bang and ends in a whimper. I'd not recommend it, unless you REALLY (I mean really!) like stealth based FPS and / or are willing to endure a terrible and disappointing second half.