Amnesia: The Dark Descent is one of the scariest games i've ever played. It's not because of the amount of monsters, either not the jump scares (Fortunately they don't exist in that game). You wanna know what one thing making this game a true masterpiece? I'll tell you, imagination.
Most of the times we are scared, because of things that are simply happening, like for example self-opening doors, windows, creepy screams from the basements of the castle etc. In that situation you immediately think that there is monster nearby, but you just realised that was a false alarm. You can also hear monster's scream, but that doesn't mean that you'll even spot him. Of course all those "warnings" aren't always fake alarms, so player have to be prepared in every moment! It's also a great survival game, you have to looking for items like oil to your lamp, tinderboxes to set fire to torches for example and other items that will help you complete this adventure!
Soundtrack is great, it fit in game perfectly. In my opinion soundtrack in every games HAVE to be good, especially in horror games. I recommend check some tracks on YouTube.
Game has also an option that allow you put some custom stories created by fans (There are a lot of cool maps to play). Also remember that some of the custom stories were making "for fun" and they aren't scary at all, but still worth to play a couple of them, no doubht about it. So if you completed the main game and You still looking for some cool, scary/fun stories to play, go ahead.
I should also say about the physics in this game. You can pickup almost every items you spot, chairs, boxes, bags etc. You can compare that to Half-Life 2, but in Amnesia this one is more advanced.
You should complete this game in 6-8 hours on the first playthrough in my opinion. I recommend looking for as many items as you can spot, since the inventory is unlimited. There is also one small addon called Amnesia: Justine, it's very short (1 hour) but in my opinion still worth to complete.
I rate this game 8/10 (Very Good), not 9/10 because it's scripted game and you won't have same fun playing this game second time since you'll remember a lot of things.
I didn't say about everything, story in game and other things, but this is my first "big" review. Also my english may be not perfect, but I did my best and i'll improve myself in future for sure!