The formula didn't change too much from Fallout 3, but the additions are well worth it!
Right now I'm playing through yet another character, (this time roleplaying as a tech junky/mad scientist who refuses to use anything that isn't technologically advanced) and I am loving it. Every playthrough has been a different experience. The difficulty in most areas is perfect and can be adjusted according to your own preference.
The game plays much like Fallout 3, and I believe it's even using the same engine, leading some people to claim it's basically an expansion pack. But this is a huge mistake. Additions to Fallout 3 would be a vast number of additional ammo types, weapons, and a few perks, completely new storyline and quests and HARDCORE MODE. Hardcore mode is for players that want to make it really tough, and it's pretty cool. You'll need to worry about dehydration, starvation, and even ammo has weight. It completely changes the game so that the way you play it is completely different. You'll start to view battles in an "is it worth it?" kind of way and you'll always question your routes, when you travel, and whether you have enough supplies to live through to the next area.
I believe this is a game that, despite the remaining bugs, should become an addition to any gamer's shelf.