Heavily divided on this one, the system driven level design is excellent, the apex of the franchise, everything is polished and works as you would expect, enemies react as it should and you interact with the world appropriately. The problem is that Ubisoft knew this, so the rest of the game is poorly done, being the grindiest as well as having the worst story in the franchise. Besides that, the whole Assasin's Creed identity is lost, as the story ignores the Creed completely in favour of building a gang (?).
It's a shame, especially after Unity returned to the roots of Ezio's Era, with the Assassin's more present and a urban setting so immersive, something Syndicate does extremely well with London and it's mechanics of building an Creed (gang in this case).
I had fun while playing, stealth is super satisfying and the game mechanics integrate well with the missions, but they only scratch the surface of what they build and is depressing seeing the wasted potential.