I think the success of XenForo will be determined by their pricing and how good their importers are due to how established forums are. The priority is understandably vBulletin and IPB, but I hope they don't leave out Simple Machine Forums, myBB and phpBB.
Many admins running free forum software are only doing so because the benefit of moving to a paid solution isn't huge. XenForo changes that and I can see a lot of people running free software making the switch, provided there is a good converter with proper 301 redirection.
I've been spending more time here lurking that I'd like to admit and that shows how great the forum software is. Will be keeping an eye on this for sure!