Yes, they are definitely worth doing! My reviews get most of my income, especially October-December. Non-fiction tends to get more views and sales than fiction, with a few exceptions -- namely the books that are on school reading lists, recommended by Oprah, etc. That said, I also do make sure they're really reviews, I've seen things labeled as reviews that are more sales pitches than anything else. I include my personal opinions and critiques, and do write a number of negative reviews. For these negative reviews, I tend to explain what was bad about it and then list a few things in my ads that might be more to someone's liking if they were considering the book/movie/item I didn't like.