I haven't played Fallout 3, but my fiance and my roommate have both played two times through.
It's got great graphics, a good plot, and tons of freedom. The good/evil chaos/order D&D structure is a lot more complicated than most other RPG games. There's so much you can do, places to go, paths to take...very interesting. I don't tend to like RPGs, which is why I didn't play it, but it was enjoyable to watch others play.
"Lurk! Withdraw! Upon them! this is the Law of the Battle of Conquest "
-Liber AL III:9