I have a love/hate relationship with the Penumbra series (and Amnesia, the demo of which I played recently). See, I love good horror, built up with good atmosphere, and these games deliver that in spades. There were plenty of times that I spent a while curled up in a corner not really wanting to go on, not because I'd glimpsed some unholy horror or eldritch abomination, but because there was a flickering light at the end of the hallway. It's not easy for a game to get that deep into my psyche, so I give all due "props" there.
However, one thing I cannot stand in games is interface screws that make the game artificially difficult. In Penumbra, there was the main character's tendency to have panic attacks if you looked at an enemy while it was within thirty feet of you while you were crouch-hiding (which made planning your way around their patrols difficult, since the combat mechanic made sure that, much like an army without profanity, you couldn't fight your way out of a piss-soaked paper bag); in Amnesia, there's the "darkness makes you crazy, and crazy makes your vision look like your eyeballs froze over" bit. They're both annoying, arbitrary limitations that reduce immersion and detract from the atmosphere rather than add to it. They're why I never finished the first Penumbra game or the Amnesia demo and why I very likely won't buy the full version of Amnesia.
And no, I didn't like it in Dark Corners of the Earth either, but then at least you could sometimes shoot or run away from what was making your vision all swirly like you'd had one too many Irish Car Bombs [http://www.drinksmixer.com/drink7774.html]... before the next three you had.