Fallout 3 is a massive experience that succesfully pulls you in and doesn't release it's hold.
The countless of choices you are faced with create so much variation, it's insane. Both the main story line and miscellaneous side-quests can be completed in a variety of satisfying ways and while the main story can be completed in less than 12 hours with a little practice, the sidequests more than double the time you'll spend playing Fallout 3.
Both the Karma and VATS combat system are well designed and can influence people's reactions and often save your life. The level of realism is outstanding, and it is hard at first adjusting to the fact that you're wandering through the wastes of DC, yet the game perfectly captures what could be left of Washington DC after a nuclear war.
From the moment you first step out of Vault 101, you'll experience a rewarding mixture of excitement and atmospheric exploration.