Well, I think the person coming closer to what the grail really is, must be American comic writer Hal Foster who, in Prince Valiant, claimed that it was the pursue of the good in your own heart, something like that.
However, most information that comes from Holy Blood, Holy Grails is not true, only based on legends or hoaxes, for instance the existence of Priory of Sion.
Personally I don't believe in the holy grail, at least not as an item of Christian lore connected to Jesus Christ. It appears way too late in Christian legends to make me believe in it.