James Patterson's book "Pop Goes the Weasel" is described as having a dull and uninteresting tone, comparable to a painful and awkward experience like a rectal exam with a cactus. The quote suggests that the story or writing may lack excitement or engagement, making it less appealing to some readers.
The comparison emphasizes how unappealing or uncomfortable the content might be from the perspective of the reviewer, highlighting their dissatisfaction with the book's quality or entertainment value.