I recently read Holy Blood, Holy Grail by Michael Baigent, Richard Leigh and Henry Lincoln; most of you can guess why. Robert Anton Wilson used it as source material for The Widow's Son (years before Dan Brown used it for The Da Vinci Code, of course) and references it in other places.
I'm not sure what to think of it. I guess it depends upon one's expectations. As historical speculation, it's a lot of fun. As a book of history, it's not terribly convincing. It's kind of a string of interesting "maybes." Despite the fact that there are lots of notes, many of the more outrageous claims are not backed up very well. For example, they don't come up with any real evidence that Mary Magdalen traveled to south France. (Chapter 11 cites "certain medieval legends" but doesn't cite any.) The claim that Mathie Paoli was "shot as a spy by the Israeli government" (Chapter 8) is backed up by nothing (Note 29), despite the fact that the death penalty is actually very rare in Israel. When the book discusses something I actually know something about -- the later Roman Empire -- it's not always on the mark. For example, Constantine was more of a committed Christian than the book gives him credit for.
Anyway, it's kind of fun, does have some real history and has some enjoyable speculation. Some of the arguments are reasonable. The Wikipedia article claims that some of the documents it relies upon are forged. I know little about that.
The front of the book has all kinds of hints about a mysterious rectangular tomb depicted in a painting by Poussin, but they never explain who was in the tomb. What's up with that?