Moz just published a study of 1000 consumers, revealing that 67% of consumers - that's more than two-thirds! - said that online reviews have an impact on their purchasing decisions.
22% of customers would decide not to buy a product after finding just a single negative article about it. This number jumps up to 59% if there are three negative articles.
Search Engine Journal offered an interesting contrast to this recently. They suggest that 4 out of 5 Yelp users check Yelp before making any online purchase and that those users are suspicious of censorship for products with no negative reviews, so it is actually good to have a few. Of course, the balance of positive reviews should outweigh the negative.
Search Engine Journal also discusses how negative reviews can affect your marketing decisions and help achieve more positive reviews in the future.