This forum is very interesting and I couldn't wait to post something, so please forgive me if this has been asked before!
I decided to build my own forum instead of using one of the prepackaged units such as phpbb or vbulletin. I've tinkered with phpbb and even wrote a few mods for it in the past, but I really wanted to offer my users something different, so I decided to build my forum from the ground up so that I would have maximum versatility. I eventually intend to add various modules that normally aren't found in forums.
Any cons to building your own forum (except for the time it takes to build it)? Mine is almost complete, it's functional right now, I just have some major tweaks that I have to work in before going live.
I based it a lot off of a popular forum that I go to on a daily basis and normally has around 600 registered users on line at a time. In the hobby, it's the biggest forum that I have found. I'm making my forum for industrial woodworkers.