Although it is good to get a free forums script, sometimes it encourages more spammers as because they see that it is free they suddenly think that you have not got much money and will think that if they spam you, then you will not have much spam protection in place. Personally I prefer a paid forums like vbulletin or something. Although you could probably get just as a good forums using phpbb or another free forums script, that is not the point. The point is that people know that phpbb is free and vbulletin is paid for. This means that there is probably more chance that people will join the vbulletin board rather than a phpbb board. They will also think you have more money to spend on advertising the board aswell.