Gregory is widely regarded as a decent historian who does her research thoroughly, but her books do frequently diverge from fact, most famously in The Other Boleyn Girl's depiction of Mary Boleyn as a naïve ingénue seduced by the king, when in reality she was not only married but had previously spent some time as the mistress of the King of France. She has, however, eschewed the Dan Brown route, being careful not to claim that her books are totally accurate and open about the fact that she is a fiction writer who will often make things up. That said, it is very important to remember that though Gregory may use the term "historian" to describe herself, her PhD is in Eighteenth Century Literature, not history. She's not exactly the type of person you should cite in a history paper.
Creator / Philippa Gregory