A great game, I only wish I'd purchased the XBox iteration so that I could have enjoyed multiplayer.
The story was great, based (as it was) on a rather interesting comic filled with conspiracy, questions of identity, and plenty of action...the only real problem that I had with XIII as a game was that the narrative ended too soon, clearly with an intent to develop a sequel, or perhaps even transition into a trilogy of games.
From the voice acting (David Duchovney providing the voice of the protagonist being my favorite) to the wonderful use of cell-shaded graphics to bring the comic feel to the interactive environment...it was a terrific game.
And, of course...the crossbows...I really loved the addition of crossbow-like weapons that allowed for stealth kills...it was easily my personal favorite as far as armament within the game was concerned.