Review Meta has published an in-depth analysis of 7 million Amazon reviews and found that "incentivized reviews," those with a disclaimer that the reviewer got the product free or discounted, skew substantially higher than non-incentivized reviews.
In fact, 87% of products have higher ratings from incentivized reviews.
The difference translates to an average of 0.38 stars higher. This may not seem like much, but that difference moves a rating from the 54th percentile to the 94th percentile.
Incentivized reviewers also tend to be less critical:
We also examined the habits of reviewers who participate in these "free product for review" programs. We only looked at reviewers who have written at least 10 reviews total. We found that on average, reviewers who have written at least one incentivized review give an average of 4.56 stars, while reviewers who have never written any incentivized reviews give an average of 4.27 stars.
So if you see a review with a disclaimer that says something like "I got it at a reduced price for my honest and unbiased review," caveat emptor!