What should I rate? The game itself, or the horrible job of porting it to the Xbox?
At some points during the game, it lags. It lags bad. It's like you're playing on a cheap PC graphics card. Hopefully conditions are a little better on the 360, if it turns out to be compatible.
The storyline of the PC version and the Xbox version are exactly the same. There's no additions, nothing taken out - well, except the fact that there's no multiplayer. At all.
This game would be an instant hit if it had some kind of multiplayer with it.
All in all, Half Life 2 is a great game, and it plays fine on the Xbox. Just don't buy it if you already have it for PC.