Nice find. A little surprised they're charging for software that's still in beta though.
As an additional incentive to adopt early, licenses bought during the beta period will have 12 months of updates and support starting from the date when XenForo 1.0 is released as a stable and supported version.
Xenforo looks pretty damn good. The biggest advantage of VB is all of the mods on vbulletin.org
Licenses don't expire during the BETA period. From Beta Release Date and Pricing | XenForo Community: