Being honest about a book is about the most important thing any blogger can do. We aren't publicists, we aren't editors, we are readers. I know I have my blog to share my favorite books and help warn people away from those books that aren't very good.
Picking up a book after reading a glowing review - then HATING it, is a terrible feeling. I feel betrayed somewhat. Then afterword, I don't even want to visit the blog anymore thinking I can't trust the blogger or their views.
I know that lots of bloggers hate to be mean and I totally understand that. I don't want anyone to be mean, but I do want them to be honest. If you didn't like it, state why! Were the characters underdeveloped? Unrealistic? Was the story just craaazy? We are all adults and we understand we don't have the same views. But be honest and don't LOVE a book when its only mediocre or you are getting free books.