From The Paris Review: W. H. Auden, The Art of Poetry (interviewed by Michael Newman):
Have you reviewed a book you’ve hated?
Very rarely. Unless one is a regular reviewer, or one is reviewing a book of reference where the facts are wrong—then it’s one’s duty to inform the public, as one would warn them of watered milk. Writing nasty reviews can be fun, but I don’t think the practice is very good for the character.