From my experience it is not as popular as other Forum Software out there or at least it isn't talked about as much as others on forum communities I visit. I came across this link that feature many popular forums that run on IPB: http://www.forumrankings.net/software/invision-power-board. Just with any forum software I think the ease of installing third party plugins are important. Also, the ability to customize the forum is critical. I've personally never used IPB but I wonder if it's worth looking deeper into. Have you ever used IPB? What do you think of the software?