I think it's a case of if you want a lot of advanced features and are going to use stuff that is only available in the paid software then by all means use VBulletin or IPB. Otherwise it's far more cost effective to use one of the many free softwares.
A comparision can be found at
http://en.wikipedia.org/wiki/Compari...forum_software though slightly annoyingly they've split the article into 3 categories (ASP, PHP, Other).