To buy a link, just to have a link, is foolish. I am not going to debate now the value of a contextual link on a niche related site. I am not even going to debate its value in a related entry.
Links in specific reviews have editorial value and they do deliver targeted traffic. But random links, like those provided by Text Link Ads, even contextual, are nothing less than… a waste of money.
First of all, these links are rented for a limited period of time. Secondly, because of the code used by the link brokers, Google knows that the links are paid and gives them no weight. Ideally you should buy such links for a short time (three to six months) and see how they perform.
Do they bring relevant traffic? Is the ROI increasing? When I say “relevant” I don’t mean “a lot of.”
Consider relevant traffic the number of visitors that spend the most time on your site, viewing as many pages and clicking on the links you want them to click.
The trend could be easily seen with Google Analytics.
If you are ready to pay for links, take care from whom you buy them. Understand that links are not created equal and, while some are great for traffic they might not perform the same from an SEO point of view.
If you buy reviews in blogs, ask the blogger to include a disclosure. Don’t try to fool your visitors by requiring a positive review and no disclosure. Ask for a honest opinion and you will be surprised to see that disclosed, honest sponsored reviews bring enough targeted traffic and pay off.