When we started this site we were on phpbb which was free an open source. We quickly outgrew that because we had so many posts and requests for new functionality. We switched to vbulletin and were very happy with that for a long time. This company called internet brands bought vb and things on the vb forums started to get negative. There were lots of complaints about their newer products. Then the sites that went with the new release V4 were complaining non-stop. They said performance was terrible. There didn't seem to be any end in sight.
That is when we switched to IPB.
VB seems to have written off v4 of their product and now their users are complaining about v5 which they released. I have always been really happy that we switched, it seemed to be a sinking ship.
The main vb team left and started a new company called xenforo. They were involved in a lawsuit which halted their development for around a year. Xenforo won (or the lawsuit was thrown out) and they are back in action. Anyhow that is neither here nor there but for anyone that is into this stuff this document is interesting. It is long but it shows exactly how bad the vbulletin ship has sunk.