Believe it? Probably most.
Understand it, able to defend it somewhat coherently to a moderately intelligent non-believer? Maybe 10%.
By "belief", I would estimate that 80+ % believe the following:
1) There is an invisible God, Jehovah, with an invisible son Jesus Christ, who have selected JWs to give them exclusive access to "the truth"
2) There are 8 guys in upstate New York who have this special mystical connection with Jehovah and have it all sorted out
3) The earth is "soon" going to be a paradise, if I don't break the rules I'm going to live forever, and once again see my dead [wife / husband / child / parent / fill-in-the-blank]
4) Anything beyond this - "just look at our website, it explains everything" and / or point 2 above