This was the big question on my mind when I was exiting the JWs. Are the GB sincere but misled or insincere evil geniuses?
After 12 years of pondering this questions, what do I think the answer is? Neither.
The GB are clowns of below average intelligence. Climbing up the ranks is a long, grueling process of total obedience and ass-kissing. Anyone with a conscience or an independent thought would be weeded out in the selection process.
The JWs are stuck with a bunch of bureaucratic yes-men who were groomed to thrive in the cut-throat Bethel environment. Sincerity has nothing to do with it. These are people who are very good at climbing the ranks and the well being of the rank and file or the soundness of their doctrine is completely irrelevant.
Asking whether the GB are sincere is like asking what the marital status of the number nine is or what the color of love is. The question itself, I believe, doesn't make sense given the context.
They're drones who were groomed to perpetuate the existence of the religious organization, nothing more, nothing less. If sincerity plays a role at all, they're sincere in their interest in perpetuating the Watchtower organization.