This must be what they call a pageturner in the world of books. You just can't stop playing it. It has one of the best story lines in a game ever, and without doubt the best story in a FPS ever. This one is going to be  hard to follow up, and it must be seen as the new standard in FPS, from now  on you'll be talking about Half-Life clones, instead of Doom and Quake  clones.