Personally i agree and disagree with both sides, there is more then enough evidence to be asking questions about what really happened, but there is some evidence to prove what happened happened.
But my real opinion is the US government knew what was going to happen, they new the terrorists where coming and what they where planning, they just allowed it to happen.
Some well placed mistakes like the fighter jets being AWOL on training and not picking up one of the hijackers as he was learning to fly a plane are just 2 examples of where they could have stopped this, but some well placed "mistakes" allowed it to happen.
This way they can control the flow of information as only a few members of the higher ups decide.