Yes, his work in exposing the nature of US interventions, the media, etc has always been very good. But the problem with Parenti, Chomsky, Zinn and just about every left intellectual is they refuse to see what's in front of their noses with this FAKE TERRORISM ploy. Not just 9-11 - they didn't see anthing funny about the WTC attack in 93 - organized by the FBI - they didn't question the Oklahoma city events - nothing to say about Spain and London bombings. So, I think its fair to say that they are vouching for the official line and misdirecting people from understanding the true depths of the problem and from reaching the necessary (revolutionary!) conclusions.
Also, they have no program for a real solution, so their endless whining about the raveges and horrors of US imperialism is inefectual. Millions of people know about the crimes of the Chilean Coup, Central America, on and on. I did a Masters thesis on the US engeneered Coup in Indonesia that resulted in perhaps one million murders. That TOO was arranged with a FAKE TERROR (Reichstag fire) event.
The only person of that intellectual stature that seems to have any clue about the central stategy of US imperialism in this period is Ralph Shoenman. I can't think of a single other left intellectual, or left party that has risen anywhere near to meeting the challenges we face today.