A lot of badly researched stuff has been written about it, but it's an interesting puzzle, if only to consider the bad arguments and find where some of them go wrong. The most convincing picture that I've found is in Lynn Picknett and Clive Prince's book "The Templar Revelation", where they dismiss the notion of a "Royal Bloodline", and even large parts of the idea of Jesus figuring in prominently, and instead say that the mystery revolves around a pre-Christian religion in France, imported during the time of the Romans, that has held on underground for centuries and centuries.
Which brings up an interesting question or inconsistency: why do they focus on the Merovingians, who were the first invaders from outside Roman Gaul to take over? If the story is true then surely the time frame to focus on is before the Merovingians appeared, when Gaul was directly connected to all the provinces of the Roman Empire and where a kind of blending of indigenous Celtic religion with Roman religion and beliefs from elsewhere occurred. The idea of Celtic religion has been done very badly to death, but in this case it's presence, which we know little about in France, is just a historical fact.
So why the Merovingians? French national pride? Marseille even had contact with the Greek world before being incorporated into the Roman empire, so why pick the Merovingians as being especially important?