Writing reviews about products that are obtained free for posting links back to the original website or the product page on social media or to the web page where the product is being sold is against Google Webmaster guidelines and such practices and putting no follow links is the only way out for linking from reviews. Companies providing free samples in exchange of reviews also need to emphasis on putting nofollow links on web pages linking to product.
There are several factors that come under reviews that affect not only buying decisions but also search engine rankings. The first thing that affect search engine rankings are the number of reviews obtained for a product. Then comes the interval that is coming in between getting reviews. The use of keywords inside the reviews also affects the search engine rankings. Adding rating button on websites especially product pages can also influence the buying decision through search results page after viewing star rating.
Google offers more reviews button on certain products having reviews on other websites as well. It is also true that local search engines line yelp also contribute to the reviews submitted on web pages. Four square also have rating options for local business. Google Plus Local reviews and rating is influenced by the number of reviews as well as use of keywords in reviews.