I can second this.

I design, import, and sell products on Amazon, and when launching a new product I offer them for free or at a deeply discounted price to people who agree to provide an honest review. From my experience, across hundreds of transactions, I can say that reviews that are solicited with free or discounted products are much tougher than those received organically. Most people participating in these review groups are so concerned about being viewed as biased that they move far, far in the other direction. If your product can earn decent ratings with incentivized reviews, you'll do very well with organic reviews.