It's good.
I'm not a crazy Fallout fan, but I do like it quite a bit. I think I played 2 and I definitely played 3. So far, this feels very much like Fallout 3, just newer. That's a good thing, a lot of people liked it. So far I have put about 3 hours into the game. I've left the vault and done a little exploring. There are a few things I like, a lot.
1) Seeing the world before the war. That was very cool, and really made the tragedy hit home.
2) Crafting. I never liked crafting in Fallout 3. It is much better now. I like finding junk, since it is easy to get that into usable crafting materials. You can do quite a bit with it too, instead of just a few special weapons. Most of my time has been playing with this stuff. I modified my 10mm pistol to have a longer barrel, better grip, and a larger ammo clip. Very cool.
3) Building. I just started to learn about how to manage a settlement. It seems pretty awesome. One of my tasks today is to clear out a gas station for materials. I still don't know if there is a real purpose behind it.
I have barely even touched the main part of the game yet. I think that's a good sign.