This is easily the best game I've ever played, surpassing even Metal Gear Solid and Zelda: Ocarina of Time. Everything is as realistic as possible. The graphics are the most amazing I've yet seen in a game. There is realistic facial animation and the physics have to be seen to be believed. The gameplay is fantastic, with AI that is actually challenging (yet fun, as the firefights with enemy soldiers are the best moments I've ever experienced in a game) and, once agaiin, amazing physics.
If there is one game I'd recommend to anyone, it'd be Half-Life 2.