Link directories or blog directories were well known in the web. They are the easiest method to get traffic to our website and also the way to get a higher Google pagerank. But I when I searched for Boonex Directory Software Barracuda, i found that boonex has stopped its distribution. They say that directory listing can cause long-term or even permanent exclusion from major search engines, like Google. I am not sure about this thing. But it is true that paid links and 2 way linking will be found out by Google and the sites will be excluded from the search results.
Google says that link backs are the important feature to validate a web sites importance. So building up fake link backs should be punished. Paid link backs will cause irrelevant search results. Here is the tool to Report a paid link to Google https://www.google.com/webmasters/tools/paidlinks