for what its worth, and similiarly to this website...I started a forum years ago based off the free versions of php based message boards. While in the beginning it worked well it things like updating and modifications/add ons where quite "manual" and forced me to learn bits of PHP...which was good for me. About a year ago or so I moved to a product that cost money...and it was definately worth it. As smulkin said, security, reliability, support, functionality, addon's, etc, etc, where made easier. You can definately apply "you get what you pay for in this situation. I don't use the same product as this site but I think our reasons for the change are on the same track. Also as smulkin said if you know the language you can have a very successful and interactive forum from the free based software as well. I enjoyed learning php and mysql but not "really" knowing it can have its problems It has helped me manage things much better though and keep the databases running clean and efficient.

sorry if i am stepping on toes or babbling to much.