Who would you say SMF's main competition is? Most people would say phpBB, since it is the biggest free forum software, but personally I think it is vBulletin, SMF is growing exponentially, and more and more people are converting, even some from vBulletin are switching over.
Its a race between phpBB and vB. SMF is certainly more feature rich than phpBB3 and as the good people at phpBB seem totally unable to create their own ACP style I would have to say vBulletin. Plus vBulletin's pricing scheme is totally unfair, I would see SMF hurting vBulletin's profit margins in the future.
