I played Fallout 3 when it came out, and just got bored with it fairly quickly. It's got that Bethesda vibe where everything is great at first, but then you start to get further into the game and everything starts to seem copy+pasted to make the game artificially larger and longer. Is it like that at all? I see it mentioned as a great game all the time, but I also saw that about 3 when it came out.
Mechanically, its almost the same game. Maybe a few UI and movement tweaks. Lots of re-used assets, although there was a bit more attention to detail when putting everything together.
Where it shines is the writing & vibe of the game. It's got a lot of charm in the world that fallout 3 petered out on. The story (esp the DLC quests) is very hit-or-miss depending on your tastes, but generally held to be at least decent.