In a study conducted by NetShops along with the University of Nebraska, product sales increased 26% as a result of implementing user reviews.
NetShops is a niche online retailer that owns about 156 retail sites including Hammocks.com and Dartboards.com. It reports revenue of well over $100 million.
The methodology of this study is interesting. NetShops essentially compared the growth rates of products with reviews to those without reviews and found that sales of products with reviews grew much faster.
NetShops uses PowerReviews to collect their reviews and claims that they recouped the implementation cost for the program within a few weeks.
In my opinion, an A-B split is a much more accurate way of determining the impact of reviews on sales. At Vitabase, we use reviews collected by health supplement reviewer NutritionalTree. Here is a report of a A-B split test done by them that shows that reviews increased conversion by 67% and revenue by 80%.
While I suspect the NutritionalTree numbers are a bit high, I trust their methodology more. Regardless, both studies show that implementing user reviews are a worthwhile investment for retailers.