Fallout 3 delivers a solid gameplay style that will make you play for hours.
The map is massive, it takes you quite a while to get from one place to another. The details are great, the feel of post-apocalyptic chaos can be felt by the player. As a player, you choose what you want to be good in, whether it's hacking computers to using melee weapons to smash some skulls.
Whatever you do in the game can affect your surroundings. If you kill a certain person, then you may have killed someone who would have been important much later ( it depends how important you want him/her to be).
If you haven't got Fallout 3 yet and are still deciding whether to buy it or not, buy it. You will not be disappointed. I give Fallout 3 a 9.5 out of 10.