It's fair to say that positive reviews can help your book a great deal, but negative reviews on Amazon can have an equally beneficial impact.
The exception is when you have only one or two reviews on your book, and all of them highly critical. Online purchasers appreciate that no book can please all audiences, and that any book reviewed often enough will end up with some occasional criticism. The big worry is when you get a particular detailed and critical review that is so devastating that it could ruin your sales.
A few of our clients have written to us in the past asking why we write negative as well as positive reviews. "Can't you see it's bad for business. So please remove the bad ones, and leave in the positive ones".
Our position on this is that while negative reviews can hurt, on the whole allowing criticism adds credibility and helps online buyers make the right choices.
For this reason, KindleBookReview.Net encourages honest reviews, and if they are negative, so be it. The good news for authors is that even critical reviews can add great value to the appeal of their books, giving them the credibility the online buyer seeks.