For the last four years, I accepted as common sense that the two remaining "Axis of Evil" nations were hard at work on nuclear weapons because I believed that the lesson we bestowed from our Iraq adventure is that the United States won't go to war with a nuclear power.
In the case of North Korea, that does seem to be the lesson they took, but what the NIE seems to indicate is that Iran took the opposite view. As I think about it, I'm considering Glenn Greenwald's "A Tragic Legacy", which documents Iran's efforts to reach out to the United States earlier in the decade, but also, generally and broadly, the notion that just because we say you're a bad guy, it doesn't make you a bad guy, and the way that people (and states) can sometimes utterly fail to fill the role we assign them.
I think the world is better off without a nuclear-armed Iran and I hope we're able to work with Iran and the world to keep it that way. The NIE is exceptionally good news, and it looks to me like it shows Iran as a rational actor reacting to regional and world events in entirely rational ways. However repulsive the regime may be, it's one we can do business with on this front, and we should.