Studied it very in-depth on and off for about three years and my hunch is this: the Merovingians/Plantards/St. Claires are not the descendants of Christ, but there's sure as heck something amiss going on.
It was interesting though.
All I remember was that I was really into it for the first part of the book, and then everything started to unravel, and by the end I walked away with the impression that it was implicitly a story about these three naively enthusiastic guys who were duped, by some loser (Plantard) living in a run-down flat, into publicizing his delusion of grandeur, or some fanciful made-up bedtime story his senile grand-mere once told him about his supposed "bloodline", or whatever.
That's not to say I think there's not "something to" all of the mysteries/secret societies/weird stuff the authors touch on in their story, though (I mean how could there not be, given the sheer scope of all the things they link together :). Just that in the end the picture seemed pretty clear: Plantard files some document/transcript with the Paris Bibliotheque (or whatever it was), the three authors go find it and follow down his primrose path, and repeat. I dunno...it was kinda disappointing when I got to this part of the story. :)