I'm going to speak freely and frankly as a librarian here: it's okay to not finish a book and still have an opinion on what content you did read.
Sometimes, we feel the obligation to read and finish each and every book we start. If that's the case, then perhaps some of us wouldn't be as avid readers as we are simply because of books that might have really disappointed us in some way or prevented us from reading a title we might have actually enjoyed. I do believe in finishing what I start, however, there is a limited amount of time given in order to do the things I love, which includes reading. If I am not enjoying a book or if there is some aspect about a book that makes me furious, melancholy, befuddled, or at the worst bored, I will stop reading and sometimes give the book away.
Off-hand, I can only think of one title that I had to put away (and I did this 2 times) before finally saying I just couldn't read it and had to give it away. If I had forced myself to sit through the entirety of the book, would I have been any better off? Possibly but in my honest opinion, I felt no need to continue reading that particular title. It just wasn't my cup of tea, and I will leave it at that.
There are some bloggers who post either "non-reviews" or "semi-reviews" of books they were unable to finish for one reason or another. I actually like the idea because they can still get their opinions out in regards to the book they were attempting to read. Maybe it was the pacing, the characters, the overall story, or perhaps even the sub-story (which might have had more page-time than the actual story), who knows? But that doesn't mean their voices should not be heard as long as it is done constructively.
The bottom line is:
1) If you're not enjoying a book or do not feel compelled to continue reading a book, you don't have to.
2) If you want to express your thoughts on why you couldn't finish the book, that's perfectly reasonable.
3) Remember, when you do "review" the book, inform your readers that you did not finish the book.
4) If you could recommend the book to readers of a certain applicable genre, feel free to do so.
And most important of all:
5) Move on and start reading the next title on your TBR list.