Yes, play it immediately.

You don't have to be logged into Steam.
Like all other Steam games you can be in offline mode to play it.
If you have payed F3 to the point of being sick of it (especially if you've done it recently) then it might be advisable to put it away for now until you feel the urge to play it naturally.
Quests and story are far superior to F3.

You owe it to yourself to play it.  The more and more I get into the game, the more and more I'm enjoying the hell out of it.

I like it more than FO3.
And yeah you can play offline.

Yes play it. The quests become way more varied and interesting than they were in FO3 in most cases. It is a very very similar game, but the story and locations make it a unique experience and there's a lot of fun to be had for fans of the previous game. Also, you already paid money and bought it, so you'd be a damn fool to not play it.

You ruined yourself to this game by playing Fallout 3 so much. This game is mountains better than "3" was to the point that this feels like the true Fallout 3. By comparison the "actual" Fallout 3 is more akin one of those weird Brotherhood of Steel spinoffs.

This game is so much better than Fallout 3.
 
It feels like the game Fallout 3 should have been in the first place.

" @Tennmuerti:  Fleshed out? you mean just leave them alone? and apparently the game has an ending just like F3 which seems stupid as everyone hated that about F3 and the DLC which will unlock it like broken steel is coming to xbox first *Facepalm* when i played xbox i didn't mind thing being exclusives but now i game on PC it's just irritating why do they have to be so selfish? i also need to find a way to turn the console off as the temptation is to much :D "