So, how do you review a book you didn’t finish? There might be a few more of these this year.
A Diary of The Lady by Rachel Johnson is an account of her time as editor of The Lady magazine. She is the sister of Boris Johnson, a columnist for one of the broadsheets, and a novellist. I got about a quarter of the way through before abandoning it.
I stopped reading because I found the writing dull and leaden. It could have been an interesting, funny story, but Rachel Johnson contrives to make it and herself come across as boring and unpleasant. Give it a wide berth.
Boudica Marginalia 
I think you've answered your own question. Knowing why someone didn't finish a book goes a long way to helping me make up my mind.
I've reviewed books that I couldn't finish and said why I couldn't finish it. The hardest part is finding something good about the book.
@Sarah, a lot of people in my book club really liked it. To the point that I thought I was reading a different book!You're both right that it's useful to say why you didn't get on with something. I don't want to be mean – so when I have to say it I will try to be factual and constructive.