I loved this book. I have no idea whether anything that the authors claim is true or even accurately researched, but it reads well and the ideas are largely plausible. I'm certain that Dan Brown derived a lot of his material from this book. But most essentially it is a gripping yarn.
I also have a soft spot for this book as I was reading it in conjunction with Umberto Eco's Foucault's Pendulum, which is not only an extremely concise and informative text on the Abrahamic cults, sects and religious orders since time immemorial but also the greatest piss-take thereof. So yes, I would recommend Hiram Key, but do yourself a favour, and if you have the patience, and read Foucault's Pendulum.