This book is not for everyone, which is why I agree with the majority of both the 1-star and 5-star reviews. If you have a limited vocabulary and don't also speak French, Italian, Latin, Hebrew, etc., then this is a very frustrating book. Likewise, if you're not up on history - especially as pertains to arcane secret societies - this book will quite likely be incomprehensible. To those with a limited education, or at least a limited knowledge base, this book almost dares you to read it.
That said, I really liked Foucault's Pendulum, in part because I happen to be keen on much of the subject matter. Two of my favorite books are "The Illuminatus Trilogy" and "The Crying of Lot 49" so I was in familiar territory here. Eco has certainly done his homework. Probably the greatest charm of all this is his tying together of pretty much EVERY conspiracy and secret society in the past 1,000 years into a vast Unified Field Theory that by its definition is deliberately a hoax, or at least a joke.
Unfortunately, the book is very uneven: it starts and ends weakly. It's 650 pages long, about 400 of which are actually interesting. Still, it held my attention, plus I learned a lot along the way.
I plan to go back and read this in a few years, and I suspect I will get a lot more out of it.