Helpful perspective from Fast Company. Excerpts:
Good reviews help, at best, incrementally, and bad reviews hurt, at worst, incrementally. They're published then they disappear, living on as pithy testimonials on authors' Web sites, or on the back covers or in the fronts of paperback editions.
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....Like news, book reviews have become crowd-sourced, with bloggers and Amazon readers leading the way. But these reviews, unlike those that appear in publications, do have an impact on sales, because they appear right next to the product being sold and persist in online perpetuity. Would you buy an electric razor at Macy's if the department store listed a bevy of complaints from dissatisfied customers next to it? Indeed, an academic paper by Yale Economics professor Judith Chevalier found that while positive reviews increase a book's sales and negative reviews dampen them, "the impact of 1-star reviews is greater than the impact of 5-star reviews."
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...But the dirty secret of book reviews is that they have always been rife with abuse, even when left to the ostensible pros. Every author knows what I'm talking about, the reviewers (most of whom have never written books) that use their pulpit to settle scores.