I've read a lot of books on the Holy Grail in my time (some may say an obsessive amount, I prefer to think of myself as thorough) from Le Morte D'Arthur to The Da Vinci Code and sadly this falls into the latter category.
The main difference between Kate Mosse and Dan Brown is that Kate appears to have done her research. Her story's relationship to legend and even actual historical events is a little less tenuous that Brown's, her grasp of English is infinitely better (not hard), as is her grasp of basic French geography (note to Dan Brown: Try looking at a map of Paris at the very least before writing about the City. If you turn left out of the Louvre you do not … oh I digress, you get the picture, this is not meant to be "Rachel slating Dan Brown again"). Like The Da Vinci Code there are times when the bad writing style (that endless use of italics to denote a character is thinking, what's that about? I am capable of working that out for myself) is so bad you want to throw the book across the floor but sadly the plot is so gripping that you have to read it until the very end.
Having said all that, Labyrinth has a lot of redeeming qualities. Firstly it's a time-slip book - half of it being set during the 13th century when the Crusaders began to turn on their own. Mosse has done a lot of intricate research into this time and really conjures up the atmosphere of medieval France as well as many who have gone before her. Secondly, the characterisation is very strong and you actually feel for all the characters in one way or another. Thirdly, it's a Grail book in which women actually get heard. If what I've deduced about Grail mythology is correct women did play an important role and I think this is what Dan Brown was rather clumsily trying to say with all his very badly misinformed Mary Magdalene information. Mosse gets the message across a little better.
In conclusion: overly long, overly hyped and badly edited, but conversely I did enjoy it and it's a thousand times better than anything Dan Brown could write