I have neither listened nor seen Book of Mormon yet. In fact, I've been kind of out of the musical scene in recent years and most of my current favorite shows date to the eighties or at least the nineties.
It's from the creators of South Park so this is hardly unexpected. I would say that their names (Trey Parker and Matt Stone) should be a red flag for folks sensitive to bad language, explicit sexual and scatological humour, and so forth.
Doesn't make it bad (I have enjoyed some episodes of South Park, though some of the humour goes off the scale even for me) but it does mean it is not for everyone.