I just installed Half-Life 2 yesterday (which is a saga unto itself) and I am floored by the game.
For starters, I have no idea how a game that looks that good (and I have all the detail cranked up) can run on this old rattler of a computer. I know that if I had a better video card and faster system, it'd look even nicer, but as it stands, it's probably one of the nicest looking games I've played on this system. The character models themselves have amazing detail on their faces and the city looks great.
Speaking of City 17, it's creepy. It's like the North Pole town from The Polar Express got taken over by a bunch of gas-masked stormtroopers.
I haven't gotten very far in it yet due to the aformentioned installation, but the 5-10 minutes I have played have been very cool. I'm just having fun throwing stuff out of windows onto the street :)