Both those who love this book and those who hate it are right.
Bud McFarlane is not Dante, he is not Dostoevsky, he is not Dickens, he is not even John Grisham on a bad day. I'm guessing that the writing is probably on a level with a lot of the trashy novels that you see around the airport (though I have to confess that I've never read any of them--one novel by Grisham or Mitchener every decade or so is about as much time as I can stand to waste).
That being said, a lot of people LOVED this book. I read it when a fellow Catholic pressed it into my hands and told me that it was the best novel she had ever read and that I HAD to read it. Even though I winced at some of the writing, I did find the book to be a page turner, and it was exhilarating for me, and I'm sure for many others, to read an action adventure story that is based on an overtly Catholic view of both the spiritual and material realms.