I do feel like I should review books that I've won, but they go on the back burner and I will review them when I can. I would actually message the author before leaving a negative review. Authors are very understanding and sometimes a book is just not someone's cup of tea. Just because you couldn't finish it though, doesn't necessarily mean that it is deserving of a negative review. I mean sometimes a book just sucks, bad writing or plot, but if you step outside your regular genre for a book, or it's a slow starter and you aren't patient, that doesn't necessarily make it bad.