I'm willing to trust the report for two reasons:
1. It was approved by the DNI, who in turn was appointed by the President
2. It doesn't make sense that Iran would have a nuclear program
I will elaborate more on the second. As I've argued before, as did the NIE, Iran is guided by 'cost-benefit analysis'. In an attempt to build a nuclear weapon, they would have to recognize, before it was completed, the program would be attacked and most likely destroyed bye either the U.S. or Israel. And Iran, guided by 'cost-benefit analysis' would realize it does not have the military might to stop an attack.
From this, one can draw the logical conclusion that Iran would only build a nuclear weapons program because it wants the U.S. or Israel to attack. One reason could be is that they need a reason to strike at Israel. This argument cannot stand because, first of all, Iran would not be able to carry out the attack. The U.S. and/or Israel would first take out major military installations so that Iran cannot retaliate. And even if Iran could retaliate, it would spell doom. A U.S. or Israeli strike on Iran's nuclear weapons would only take out nuclear and military installations. Iran - if they could - in retaliation,would be sure to strike civilian centers. The U.S. and/or Israel would then wipe Iran 'off the map'. And if Iran is basing its decisions off of 'cost-benefit analysis', they would realize this, and would never retaliate.
The only other reason Iran would build a nuclear weapons program, other than looking for a fight (which we just proved makes no sense), is that they would be looking for a North Korea-like deal. They would expect some economic or geopolitical aid in return for the shutdown of weapons.
Iran is different from North Korea. In North Korea, they had already spent a huge amount of money developing their weapons program. Iran hasn't. North Korea wasn't planning from the beginning to ask for economic aid in return for its weapons program. It was planning to use its weapons program as a threat against South Korea. Also, North Korea is ruled by a crackpot dictator who loves attention. If Iran is basing its work on 'cost-benefit' analysis, the cost is much different from North Korea's cost. The benefit would not outweigh the cost in Iran's case, unlike in North Korea's case.
Iran could be looking for 'geopolitical aid' in return for the shutdown of its weapons program. But once again, the benefits do not outweigh the costs. The 'geopolitical aid' Iran would receive from the U.S. would almost definitely be in Iraq. Iran would be given more influence there. But the geopolitical cost of even having a nuclear weapons program would be huge.
And remember, MOST IMPORTANTLY, the U.S. never has to give Iran any aid at all. The U.S. could just say, "Nope, we're not giving you anything," and give Israel the go ahead to blow up Natanz and other nuclear facilities. That would mean Iran gets a negative return. They spend all their resources building facilities, and in return, get their infrastructure destroyed.
Is it worth the cost? No.
On My NIE Analysis
The Significance of the NIE
Why The NIE Should Be Trusted
How Will the NIE be Used?