One for the 'insanse conspiracy theories' category. Having not read The Da Vinci Code, I've no idea where the comparison stands, but... it's a slightly silly conspiracy thing. Being an Umberto Eco book, all the characters talk as if they've got higher degrees in the Arts, and carry around thesauruses (thesauri?) that they use at every opportunity. I pity his poor translator. After a while, I gave up, and let the words wash over me, which worked quite nicely for the explanations of the overly-complicated and non-sensical theories.
The nearest book I've read, in many ways, is Robert Anton Wilson's Illuminati trilogy, which I read on the recommendation of The New Hacker's Dictionary before realising that it's basically Eric Raymond's personal choice, and that Eric Raymond is basically mad. The Illuminati trilogy is fundamentally a big book of sex, drugs and conspiracy theories in a self-referential '70s counterculture kind. It turns out pretty juvenile. In comparison, Foucault's Pendulum, while self-referential, does it in a rather post-modern kind of way, and its educated tone helps it get away with it.
Perhaps a better comparison is with Sophie's World. While that book used a story to rather neatly encapsulate a Philosophy 101 course, and could thus keep a nice divide between history and story, the conspiracy theoretical nature of Eco's book blurs the line between the factual (or rather, at least long-standing theories) and plot devices. Which, of course, is pretty much self-referentially covered as a major part of the plot.
So, did I enjoy it? Yes, but not as much as I thought I would, from halfway through. It's not a twisting and turning storyline, so much as an enjoyable literary ramble.