I tried various forum plugins on my testsite, among which the free Mingle Forum plugin. To my surprise, categories and forums I created in the FREE Mingle Forum plugin also appeared in this "Forum Server" plugin. As a matter of fact, the entire look and feel was so very similar, I was somehow confused a bit and thought I re-enabled the Mingle Forum plugin by mistake (but it was uninstalled).
So, seems like a bit of a scam to release a commercial plugin that has a totally similar free basis. It kinda sucks actually.