If you didn't like a book but won't post your review saiyng that, isn't that as bad as lying in your review? Because omittance is a kind of lie. You leave your reviewers in the dark.
(Of course there are reviewers who prefer to avoid confrontation and state in their review policy that they will only post positive reviews, and that is fine: they have been honest with their readers from the beginning, the readers know that all they gonna see there are positive reviews. In my opinion this case is different than the one this post is about.)
Sorry, I'll try to wrap this up as I've been rambling on for a bit now: my bottom line is that as book reviewers our readers trust our taste and judgment, we owe them and ourselves to be honest and truthful. If you don't like something say so, just do it politely and in a constructive, objective way.
What do you think? Do you write negative reviews or if you didn't liek something prefer not to write a review? Has an author ever asked you not to post a not so stellar review about their books? Tell me your thoughts, I'd love to hear them! :-)