Not for the faint of heart or the weak of mind.
For me, I at first did not think much of this game. I bought it when it first came out, played it for about a half hour and then it sat on the gaming shelf for a few months before stumbling upon it again and deciding to spend some actual time playing it. When I started playing it again I found I couldn't stop and kept playing and working through each level up to the end.
Thankfully at the beginning you are given a short tutorial to help ease you into the game and orient you to the game mechanics itself.
Overall this is not an action game but one of stealth and strategy. You cannot just walk up to enemies in this game and expect to come out alive. In this game they will most likely win against you, especially if you are facing more than one opponent. You will have to think, you will have to plan, and you have to utilize your environment in order to kill, execute, and advance.
As mentioned in the beginning, this game is not for the faint of heart. You will kill, and sometimes even get brutal in your killing. This is where the game draws its controversy from: the brutal executions, all of it uncensored.
In this game you have 3 different levels of executions: Quick, medium, and brutual. And with each level of execution comes the strategy. Do you quickly execute the enemy or do you want to really stick it them (sometimes literally). However with these 3 levels of execution comes the time it takes to take action on them. The quick can be instant, the medium maybe 5-10 seconds, and the brutual up around 10-15 seconds before it can be done. All this while you have to keep your enemy unaware of your presence because if they see or hear you, they will target you and you will be unable to perform said execution.
The AI itself can be a mixed bag at times. Sometimes they'll see you in the shadows, even though this is considered the way to "hide" your character. At other times you will at the edge of the shadows and be within arms reach of a person looking directly at you and they do not see you.
As for the levels themselves, they set a great pace and feel (think "ambience") for the type of enemies you will be facing. You will fight through urban environments, a prison (or mental institution), a zoo, a run down house and many other areas on your quest to find the answers to your situation.
Is this a murder simulator? Nope. I view it like I would a horror movie, except you are the star. And in the story of this game, you will want to know why you are being put into this sick game and who is putting you there.