What add-ons are worth it?

I've just finished my second playthrough of the base game. And I don't want to put the controller down yet, but I'm not sure what to get. For those of you that have played them, what are the better add-ons to get?

The price tag through Xbox is such that you're paying for almost an entire game. And if you get all three, that's a hefty penny. Are you getting three full games? Were you getting only 20 hours of additional gameplay?