Sorry for bumping this topic, but according to the "Forum Software Reviews" website, PunBB is suitable for "medium" communities, it can be up to just 10,000/100,000 members, and the same number for posting messages. It's a little limited, because when that certain number is reached, the forum starts getting errors/bugs, and users need to touch complex code without even having knowledge of technical things, excluding those with knowledge in that kind of things. A good forum wih problems is THIS ONE using PunBB: http://forum.youhosting.com/ Also the blue colour doesn't fit at all with the "Oxygen" style, it can be much better in my opinion.
Sorry for my bad English.