The Bazaarblog has some very interesting numbers on how users value reviews.

* 63% of users prefer sites with ratings and reviews (eVoc Insights)
* 48% of shoppers seek reviews before making a purchase decision (eVoc Insights)
* Conversion of customers buying a single product nearly doubled after the product received a five-star review, going from .44% to 1.04%. (Marketing Experiments / Shopping.com experiment)
* 59% of users considered customer reviews to be more valuable than expert reviews. (BizRate)

This are all interesting numbers. But the most relevant is:

* Users who contribute product reviews visit a site 9 times more often than average users, and they remain twice as loyal and buy twice as often. (McKinsey)

What makes users comeback is the desire to be part of a community. In doing this they are add something invaluable to the conversation!