Hurrah for this game! I recently began playing it after my severe disappointment at Two Worlds, and it is totally worthwhile and enjoyable. The actual "game" part of Assassins' Creed is good and it is easy to learn the interface of the controller, but the storyline itself has thus far taken a back seat to my wanderings of the beautifully depicted streets of medieval Damascus, Acre, and Jerusalem. The simulated environment and believable artificial intelligence of this game--combined with your character's ability to almost superhumanly climb on virtually any wall or tower--has occupied most of my attention, and I have found myself spending most of my time wandering the streets, exploring the rooftops, and testing out just how much weird and violent behavior the city guards will tolerate.
My only regret thus far in playing is that my TV is too small and thus cannot display the full potential of the stunningly articulated textures and graphics of the game.