From what I've heard, Xenforo gets very mixed reviews.
Some people support the Facebook integration, easy theme selection and customization, and the Search-engine Optimization (SEO) makes finding things from your forum on google easier, increasing traffic.
On the other hand, people dislike the Facebook integration (namely, people without facebook) and it is said to have problems with cookies. For example, users on Google Chrome can not stay logged in.
But the installation is said to be amazingly easy.
Buying the software is $140, and removing copyright information on each page is an additional $250, so if this is for something professional you will need to pay ~$400, not sure you want to put that much money into this. Of course, there's always 'unsupported' options that we are not to talk about
Finally finding about some of the bots that are flooding the users online - bytespider apparently is a huge offender here - ignores robots.txt and comes in from a ton of different IPs
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.