You bring up a very good point. I like reading reviews before I buy things, but with all reviewers basically living off the free stuff they get, I do worry that there is a conflict of interest. I don't imagine they get a lot of email like "you gave our thing a positive review while the rest of the Internet appears to hate it. Your reviews are not very thorough so we're not sending you review units anymore."
I bet if you do write a lot of negative reviews, the UPS guy doesn't visit your office as often as the author who writes a lot of positive reviews.
Pure speculation of course.