Search engines use complex algorithms to determine the relevance and quality of a website’s content. One factor amongst many is the incoming link. These are seen as recommendations and therefore any paid link (recommendation) disguised as an organic link does nothing but corrupt the quality of search results.

Please find a statement from Google regarding this practice, which confirms their intent to stamp out this practice and to penalise those involved:

http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66736

To preserve the quality and integrity of search engine results we encourage all site owners to improve their search engine ranking through unique, informative content delivered via a well coded website.

If you are wishing to pay to improve your search engine ranking, we recommend a service such as Google Adwords. In many cases this will offer you a much better return on investment when compared with paid link networks.