Pros: Great graphics and music, compelling story, great mod possibilities
Cons: Not particularly difficult to beat if you don't get scared due to somewhat simple puzzles.
First of all, note that I bought this game via Steam, as opposed to a physical copy of the game.
Amnesia: The Dark Descent is great in a lot of ways. I'm actually not normally into horror games: on the contrary, the main reason I bought the game was because of its relative lack of intense violence. There are some really intense scenes, nonetheless, not only through the monsters (including a few chase scenes) but also through the extremely dark storyline.
Graphically, I find it amazing, especially for such a relatively-small, independent game company like Frictional Games. The music is also very chilling, though it did surprise me that some of the pieces are somewhat electronic in contrast to the castle-in-the-19th-century setting.
Anyone who's a fan of the Myst games may also enjoy Amnesia: there are several puzzles involved, although almost none are as sophisticated as in those games.
The game is also surprisingly readily accepting of mods for it. The reason there are so many YouTube Let's Plays involving the game is partially because many people have made horror games of their own with tools Frictional officially provided. Therefore, if you wish to pursue game design, definitely check out that aspect to the game as well.
All in all, this is a great game for people who are into horror but not in a relentlessly violent fashion like for Doom 3. The easily scared should probably stay away, but otherwise, it was worth buying to me.