greg said that PunBB couldn't handle the load of a large forum.  I seriously doubt it - it's most likely bad configuration or junk plugins (actually, I believe the site was getting more hits on PunBB than on vB).  But I guess it might not be worth the time to fix things up than get a vB license, depending on how much income the site generated.  But then, vB is perhaps one of the most server intensive systems, whereas PunBB is perhaps one of the least server intensive.  Go figure.

Not used on his other sites, so probably the cool factor, as said.