Good, but Oblivion was much better.
However, despite how awesome the world is, there are several things that I feel keep Fallout 3 from being among the greatest games of all time. First: third person combat is a joke. I'm not even sure why they bothered to add it - it's that bad. However first person movement is not the greatest either, and often times the weapon animations and combat sequences leave me with a very artificial aftertaste. From the very beginning, with your 10mm pistol that sounds like someone cracked an egg, to the later stages of the game when you uncover new weaponry, you will notice that combat, be it in first or third person, does not have the seemless mechanics that we see in games like Crysis or CoD4. And while many may overlook this because the game is technically a RPG, I often found that I forgot I was playing a RPG due to the fact that the game is played like a FPS.
Overall, Fallout 3 is definately worth getting, and as long as you're willing to overlook some of the flawed FPS elements of the game - you're going to have fun playing it.