Here is an interesting analaysis of Congress's use of NIEs, by the Congressional Research Service, a NON-PARTISAN branch of the Library of Congress. It discusses in detail what Congress gets and who in Congress gets it, and the details of the 2002 Iraq NIE events.
The intelligence process, however, is not an exact science and, on occasion, NIEs have proved unreliable because they were based on insufficient evidence or contained faulty analysis. This was demonstrated in the NIE produced in 2002 on Iraqi Weapons of Mass Destruction, parts of which were significantly inaccurate. NIEs can provide insights into the likely effects of certain policy approaches, but they are not usually made to take into account the details of planned U.S. diplomatic, economic, military, or legislative initiatives.