I just recently purchased Fallout 3 because I've been looking for something to suck up all of my free time. This game is amazing, but everyone who played Oblivion should know that Bethesda makes some awesome games. So far I've spent about 12 hours just walking around the landscape doing random things unrelated to the main quest and not once did I get bored or annoyed at anything major in the game.
The one thing I would like to note is that while the game is awesome, the combat could be better. The VATS targeting system helps greatly when killing enemies and almost makes the game too easy when you can just automatically blow an enemies head off. When you try to play without using the targeting system the game gets much, much harder and a lot less fun as you'll be either running for your life from the tiniest enemies or out of ammo using a baseball bat or lead pipe to kill characters with guns.
So far I think this is definitely in my top 10 games of all time just because I can spend so much time doing absolutely nothing related to the story and feel as though that in itself is a great game.