This game is incredible. It has good graphics, characters you easily warm to, a great storyline, amazing gameplay, freedom to do things your own way (you can either kill almost everyone you meet or no-one at all, you can use force or stealth and people can be killed or incapacitated in a variety of clever an creative ways), and brilliant replay value (I think there are three endings you can get, as well as a variety of ways to complete your missions).
I haven't been as obsessed and addicted to a game in a long time. The closest thing I can make to a negative comment is that I'm glad I completed it quickly because the sleep deprivation was really starting to hit home, especially at work.