It was alright, reasonably entertaining, just nothing to write home and all your friends about. I did enjoy the history and scenes from southern France, but I wish the author had kept the story in the 13th century and skipped the stuff from 2005. I still haven't gotten around to reading the DaVinci code, so I can't make comparisons as other reviwers have.
If you want to read more about this period of french history, the cathars, knights templar and the grail, try Elizabeth Chadwick's Daughters of the Grail. I could not put it down. It is currently only available used in the US, but the author has recently rewritten and republished the book in the UK and is available on Amazon UK now. Hopefully it will be available in the US soon, as the used paperback is quite spendy (although worth it).
If you're set on reading this, save your money and get it from the library first. Then if you absolutely love it buy it. JMHO.