Sales are up at the Petco web site since they signed up with Bazaarvoice to collect reviews of the products. In the NY Times, Bob Tedeschi has an excellent overview of the trend, but the only cold hard numbers are from Petco so far. I expect that selling products online will now often require some customer-generated info accompanying the product, or being accessible to the shopper.

NY Times Help for the Merchant in Navigating a Sea of Shopper Opinions, 2006-Sep-4, by Bob Tedeschi

Earlier this year, the site devoted entire sections within each pet category to “top rated” products. Shoppers who browsed products that way purchased at a 35 percent higher rate than those who browsed assortments arranged in the traditional manner. And those who bought from the top-rated sections spent 40 percent more than those who did not.
“I think we’re one of the first, if not the first, to create a primary shopping experience that’s driven by the voice of other shoppers,” Mr. Lazarchic said.