When Is It Okay to Not Like A Book - And Say So...?

Like many of you, I was a reader before I became a writer. I love books. From the time I was old enough to stand on the small stool in front of the counter and scrawl my name, I had a library card. My mother and I frequented the adult library and the children's library. I'm sure we spent hours in both places. Rows upon rows of shelves stacked high and full to overflowing with BOOKS! Books of every description. Books with covers to die for. Books with nondescript green or blue covers, leaving you to wonder about the story inside. Old books, new books, every kind of book you could ever want. Paradise!
And the smell, remember the smell?
I haven't been to the library in a long time. In our digital age, I confess to downloading most of my books. I think I shall have to take a trip into town, just for old times sake.
Ah, but I digress.
Back to the matter at hand.
As a writer, I do of course still read. The problem is, there are SO many books out there! Good books. Books recommended or even written by people I know. Writer friends that I interact with and sometimes see at conferences.
We are a supportive community. We buy each others books, tweet about them, blog about them and host the author for an interview or two.
But we also need to leave time for our own writing. These days I need to be very selective in what books I choose to read. I want to enjoy the time spent away from my own stories. I don't want to waste my time. I simply can't afford to.

If a book does not grab and hold my interest, I won't finish it. I don't have a driving need to find out what happens in the end or if the story ever makes any sense at all.

Usually, if I have enjoyed a book, I'll post a review up on Amazon and Goodreads. As an author, I realize the importance of this kind of publicity. I love getting reviews, especially when they're from readers I have absolutely no connection with, who have somehow heard about my book and picked it up. I'm usually pretty generous in my review as well, even if there were small things I didn't like with the story. I'll give it a four. If I love a book, I'll say so and tell you why. I'll give it a well-deserved five star rating and tweet about it to all my friends! I love finding books like this. It's exciting for me to see a brilliantly written book that has captured me from the get-go and provided entertainment. I haven't wasted my time, and I'm glad.

But what happens when you don't have that experience?
Do you write a scathing review of the book and list every single thing you hated about it?
Or do you say nothing?
I honestly don't know so I'm asking you.

I picked up a book recently that unfortunately, I didn't like. I'm in the minority, it would seem, judging by the reviews. I'm really disappointed because I was looking forward to it. I loved the first book by this particular author and just assumed I would love the second. But I didn't. I haven't even finished it, and I'm probably not going to.
So what do I do here? Do I pretend I loved it and write a review that really doesn't reflect my true feelings?
Or should I be honest?
Of course the third option is to say nothing at all. I don't enjoy hearing from a reader that hasn't liked my story. Nobody wants to be told that their baby is ugly. After all, beauty is in the eye of the beholder.
We all know the publishing world is subjective, right? Surely I am within bounds to post my not-so-flattering opinion?
I'm not sure. In this case, I think I may choose to go with the whole if you can't say anything nice don't say anything at all adage... I didn't receive the book in exchange for a review. I am not bound by any means to put something out there, so I probably won't.

So now I'm asking you. What do you think? Have you ever had to write a review of a book you didn't like, knowing you'd probably run into the author at some point? How did you handle it?