Incredible.
This game really feels like the natural successor to the Prince of Persia trilogy, as it encompasses a lot of the aspects of it's predecessors (e.g. wall running, climbing buildings and sword battles) but also expands on them and makes them more of a natural occurrence rather than something that is used only when needed and, in Prince of Persia, could probably kill you if you used it at the wrong time.
The huge landscapes are detailed down to the tiniest cracks in the rocks which your character, the masterful assassin Altair, can use to climb the highest of buildings.
Unlike in Prince of Persia, where the ledges and things to grab on to occasionally feel out of place, in Assassin's creed every crack, every decoration and addition to the landscape which helps your character travel about the game, feels totally natural and perfectly placed to add even more depth to the graphical aspects of the game.
The characters are also very well modeled, and although your character's clothes can sometimes look odd in the way they move or their edges, he more than makes up for it with the intense detail of his head, hands, amour and even his hood and weapons!
The wide landscapes also mean a lot of characters, from religious/political preachers who seem to love the fact that there are thousands dying in the crusades even though they are on the side of those being attacked to the poor beggars who follow you around so persistently everyone seems like a natural character for the time and place you are in and everything feels so real.
This is definitely a must have game that any collection needs.