I recently setup a product review system in a clients online store: www.YDS.com.au.
Customers took a while to review and comment on products so we decided to offer a incentive for any customer who posted a review, good or bad, we would give them a 10% discount on their next order. So the process was, after a customer placed an order, 14 days later the site would automatically send them an email asking them to review with a 10% off coupon code. This drastically increased the amount of reviews we received but also increased the customer return rate as they now wanted to spend their coupon.
Over the next few months we noticed products with reviews got a much higher ranking on Google than those without. The star rating also started showing up in the organic listing. I think its a clear case for anyone contemplating whether or not to have a review system on their site.