I didn't believe the hype at first. Boy was I wrong.
Firstly, this is done on the Unreal engine, and performs beautifully. Everything you expect to happen happens. No surprises, no glitches, nothing but you and your skill. Oh, and you better have some for this one.
The voice acting is excellent, and frequent. You'll be constantly eavesdropping on guards in conversation, their comments on what's going on, they even tell "you" that you look ill if you possess a guard near other guards. Which is a ton of fun, by the way, and gave me fond memories of Oddworld.
The story is pretty good. Going into it blind I was very pleased with the twists. They were a little predictable but fun to experience nonetheless.
Graphically, this game is a 10. Environments, character models, animations, visual indicators in the UI, everything is pretty. There's replay value in this part alone.
But the real replay value comes from the gameplay. First off there are many, many, many ways to complete a mission. The game reminds you of this at one point, and it's not kidding. So don't stress about what you want to get too much insofar as your "area unlocking" powers. You'll find a way.
Second about gameplay, is, well, combat and stealth. Both of these seamlessly intertwine, and both tend to be just as difficult. You can either spend a minute studying guard path routes before you Blink behind a guard to quickly choke him out and carry him away seconds before the other guard turns around, or you can risk getting your ass handed to you by a mere handful of guards that you would think present little challenge to a badass assassin like you (protip: they do). Or you can Possess one, walk him away, pop out of his body and strangle him there--or you can just ignore them entirely and hope no one sets off an alarm to bring them over to you. Or you can rewire that nearby Wall of Light to... yeah. The choices in this game will leave you thinking, but they're all really fun to play.
Also, the Heart. This is possibly the coolest item in any game ever. First, it points you in the direction of nearby Runes (upgrades your powers) and Bone Charms (give you minor but useful bonuses). Second, if you right-click, it gives you a narrative on whatever you were targeting in a mystical (and sultry) female voice. You can learn snippets of characters' backstories this way that they never tell you otherwise. And of course, there's a pool of generic options for pointing at unnamed guards and civilians and the like.
One thing in particular I really enjoyed is how the game makes you feel guilty for killing certain people who seem generic. They may be generic to you, but they were important to someone else!
Lastly, the difficulty curve is perfect. By the time you reach the final mission, you will feel like a badass. Until you get *into* the final mission. I'll admit though, one mission was a little easy, but hey, it may have just been the path I took that seemed really obvious to me but maybe not to others.
Now, for the cons! Of which I have few.
Guard dialogue gets a bit repetitive. Not much variation in their generic dialogue, which also leaves some of them sounding kind of... off. Here are some examples: when you hear "things are bad, could the city get worse?" you'll eventually already brace yourself for hearing "never doubt it" (a rather grim outlook, but all right), "indeed, I think so" (which is it? an emphatic yes or a possibility?). Just kind of an annoyance after a while, but mostly since because it plays so much.
In all, this is a game worth playing, period.