I am considering moving my VB 4.2.0 forums to Xenforo. My primary reason for migrating from bbpress to vbulletin was spam and vbulletin has miserably failed at it. vBulletin 4.2 is crap and I have already wasted my $285. After installing vBulletin on my VPS. My daily CPU usage has gone up by 200% and bandwidth by 100%. The worst part is that I have to shell out more money in order to get support from VB. Later I found that VB's mail script is easily hacked and 1000's of emails are being sent from my server.
I hope above experience expresses my concerns. I want to make sure that I do not repeat the above mistake and therefore want assurance that XenForo would be able to control spam and keep the CPU+Bandwidth usage under control.