A few serious points against Moore's credibility are overshadowed by a lot of dull posturing, unconvincing argument and unpleasant interviews. It's nothing like as good as the films made by the man it maligns.
Manufacturing Dissent comes across less as an expose of a polarizing public figure than as the realization of a personal and not especially interesting grudge against a once-admired colleague who has moved on to bigger things.
The big question you have to ask yourself is: do these dishonest work practices actually undermine the message he's trying get across? The answer is probably not... because Moore is already preaching to the converted.