Reviews are like flyers in your local travel agency - some have no impact on you whatsoever, but some make you want to visit a certain place so bad that you literally can't wait to reserve a flight and a hotel. Writing a review is not about convincing anyone to do anything. It's about giving some honest opinions about a given product and listing its pros and cons.
Well OK, maybe not the most, but it's still pretty counterintuitive. I don't intend to keep you hanging, so let me just go ahead and tell you what the advice is: Instead of looking for an untapped niche, enter one that's already crowded. Why? Here's the answer.