The last few books I've tried to read have been so boring that I didn't even finish them. I can't write reviews on books that I don't finish. I don't think it would be fair to my readers if I did.
So, what do you do when a book is so boring?
Stop reading! There are too many fabulous books out there that I would not waste my time! Life is too short!
Don't give up on a book that's boring. I'd still be inclined to write a review. Remember, your readers want to know your opinion of the book, so even if you don't like it, you can still write a review. It may not be positive, but it can still be constructive criticism. Just say what you didn't like about it and if you gave up, why. Invite your readers to tell you what they thought of the book, if they've read it. Some may agree and others may not, but it could get an interesting debate going!?!
I've reviewed books where there have been things about them that I didn't like and I've just explained why. I've also made the author aware that I'm going to make those comments beforehand and I've never received negative feedback. Most authors will tell you that even the negative comments (if worded politely and as constructively as possible), will help them to develop as writers.
I usually flip through the book just to get an idea where the plot is headed - who knows, perhaps it will surprise me, though that's usually not the case. There is only one novel where I genuinely haven't cared how it ended (or even whether it ended), and that book was Cold Mountain. It took me a week just to get to the halfway point and by then I was sick to death of it.