With this game, Ubisoft goes back more back to the Assassin's roots. The new engine, the cities are alive and you can well go into hiding in the ground. The characters in the cutscenes look a lot better than ever before.
However, the game is still propagated bug infested and suffers from framerate drops.
Who Assassins Creed games even before Black Flag liked cost will come here on his own, if you can live with a bit unfinished acting technique