Unsurprisingly, this brief analysis of how reviews influence sales on Amazon equates quite well with my purchasing behaviour; I wouldn’t feel comfortable buying a product with 100% positive reviews unless I knew personally that it was awesome. And a product with less than 15 reviews or so? Forget about it.
[A] handful of bad reviews, it seems, are worth having. “No one trusts all positive reviews,” [John McAteer, Google’s retail industry director,] says. So a small proportion of negative comments—”just enough to acknowledge that the product couldn’t be perfect”—can actually make an item more attractive to prospective buyers.
The sheer volume of reviews makes far more difference, according to Google’s analysis of clicks and sales referrals. “Single digits didn’t seem to move the needle at all,” says Mr McAteer. “It wasn’t enough to get people comfortable with making that purchase decision.” But after about 20 reviews of a product are posted, “We start to see more reviews—it starts to accelerate.”