just my two cents here, since it will vastly differ from what most people are saying.
despite contributing quite a bit to the big xenforo thread folks have linked to (see my posts and screenshots there), i was NOT able to get the xenforo importer to work, despite testing across multiple versions and even with a vanilla install of a fresh xenforo system. while i’m not a ruby programmer, it’s pretty obvious the importer is broken, and no one is willing to work on fixing it.
i gave up because despite my extensive input, the consensus was ‘pay someone to do it’ rather than anyone volunteering to fix the importer and take my input seriously.