I found a very relevant subreddit and offered to send a sample to anybody who was willing to leave an honest review, up to 10 total. I got very enthusiastic responses, and sent out the samples.
Every review that I received from this group was five stars. I at no time asked for five stars, and was very explicit about asking them to be honest. Maybe my product is just that good, but I suspect there's at least a little bias there.
More to the point, every organic review I've received since has been five stars, which leads me to think there's a crowd effect going on.
I think what I did was a good deal less scummy that what ReviewKick et al do, since they explicitly find "good" reviewers for you, and the reviewers aren't necessarily knowledgeable about the product or niche. That said, even my targeted approach yielded a lot of bias.
For what it's worth, this exercise doubled my sales. I now effectively earn the equivalent of a full time minimum wage job through one tiny niche product, and an hour's work every couple of months.