I'm not foolish enough to say anything about the actual accuracy. The CIA has a pretty bad history when it comes to picking up the important stuff before it happens. But it is the political impact that is important here.
In a rational world Bush would be forced to back down if the NIE produced by his own intelligence service didn't back his position. How can he claim to be working from valid information if the governments own experts don't see it?
Bush, naturally, won't back down at all. And is liable to come back with something like "We have to invade now, before they actually have nukes."
But it will mess up his ability to cow and/or confuse other governments and Democrats into backing him.