Dishonored is a fantastic game, one of the best in the stealth assassin genre. It's not for everyone though. I think its vastly improved if you get taken in by the narrative and the world design which are elements that just don't matter to a lot of gamers. Still, the level design is excellent and as mentioned the choice between "silent stealth assassin" or "FREDDY FUCKING KRUGER" is a big selling point. The game really allows for both play-styles and also significantly changes the last level depending on your approach throughout the game. That and the blink power as well as the generally vertical nature of the level design gives the game a different approach than the majority of stealth focused games out there.
Saying that, for as much as I love the first, I've still not got round to the sequel which also happens to have Stephen Russell, the voice of the protagonist Garret from the Thief games (another series I adore) coming in to voice Corvo. That probably won't mean much to most but for a nerd like me thats about as perfect as it could have went.