We're replacing vbulletin 3.x with the latest ipboard and so far ipboard is working fantastic in our private alpha forums. Was considering xenforo and a couple others but am very happy with ipboard so far.
I'd hope anyone going with any software free or paid would work very hard to make it as successful as possible.
Personally though with the routes vBulletin has gone down I wouldn't consider them for any project, if I wanted a commercial solution the only options on the table would be XenForo, IPB & possibly Burning Board (as the v4 sneak preview looks quite impressive).