Overall, whatever you think of the ideology of the authors, they know what they are talking about. Peter Jones has a Princeton Ph.D., Garlow's is from Drew. It is also written in a non-technical, highly accessible style. Another appeal, for me, is that it deals with teh broader cultural issues (Gnostic world-view) as well as just the details of the Da Vinci Code.
I also appreciated Ben Witherington's book on the topic, and D. Bock, both world-class Biblical scholars.