There are tons of credible disputes about the accuracy of his "videos":  CONTEXT & heavy editing to misrepresent.  Not how he obtained his video but how he edits which goes to the heart of accuracy.

O'Keefe  paid $100,000 to settle a lawsuit from a former ACORN employee:  The lawsuit alleged that O'Keefe and his associate filmed Vera in the San Diego ACORN offices without his consent, a violation of California law, and portrayed him untruthfully. The video was later heavily edited and published on conservative mega-blog Breitbart.com, making it appear that Vera had conspired with O'Keefe to smuggle underage girls across the Mexican border, when in fact Vera had immediately contacted the police after O'Keefe left his office.