I was actually with a group of reviewers who did this. People with products would approach the ladies who were heading up the group, and based on our (reviewers’) interests, they would either assign certain reviews or reach out to ask if we would be interested in a review. It was a win-win: the person whose product was being reviewed got some mentions on the website and had a review they could post, and the person reviewing the product got to try out a new product. I actually purchased a couple of higher-priced products as a result of getting free products to review, so the seller made some money AND got some good press.
Bruce has some sound ideas about how to evaluate these offers. Always trust your instincts – if you get a weird vibe, chances are there is something amiss that you are picking up on, and you should pay attention to that.