I've actually pondered this very question for some time. I've come to the conclusion that there are some that feel it is BS, a few who do not agree with all of the teachings and maybe a few that actually believe they are God's gift to men. Let me go into a little more details.
The reason I think some if not most think this is complete BS is because you do not get that high up in any organization without thinking outside the box and careful planning and goal setting. These men are not dumb and they do not follow their teaching of "thinking inside the box only". Think about this too, we know maybe 10-20% of all of the WT secrets and dark history, these men most likely have had access to it all and have been told it all. Knowing what little we know about their dark history and the darkness that goes on inside turned us off, can you image believing it is the truth knowing everything? I highly doubt it...
Now, my reason for some not believing in all the teachings is as follows. Franz made a comment in his book that even if individuals did not agree with the current understanding they had to be "in union" and only bring out the current teachings in their talks. They set an example out of those who did not follow this. I'm sure many disagreed with the generation change, the internet, 1914 I can't even imagine any of them believing that etc... Remember, there are those on the GB that probably did not vote in favor of those teachings and have to act as if they believe them just as in Franz day.
Last but not least are those who may believe 100%. I find this one hard to believe. These men are thinkers and I can only imagine the thought and ways they came up with to make the "generation" term fit to their teachings vs what the bible says. I would be very surprised if there were those on the board that had 0 doubt. You would have to be naive to not question a thing at that level.
Just my 2 cents, at the end of the day we all sh** on a toilet and it's no different with those men.