I believe that it's extremely unethical to accept payment for posting false user reviews. What I found even more disgusting was the action of clients posting positive reviews of their own products under the names of different people. In the future, I will be much more skeptical when viewing user reviews on popular product websites. There is just no way of knowing whether or not the people posting them have been paid do to so.
Nevertheless, this kind of behavior seems like it will be tough to monitor. How can you tell if someone has actually used the product and is being honest about it? Additionally, since user reviews are being used more and more with the influx of online shopping, I see no end to this user review abuse.