I was actually worried about posting here since any opinion against ProBoards seems to be shot down so I'll tread really carefully - all I'm going to say is when I used it (v4) I believe back in '05/'06 and when I moved onto phpBB, IPB, MyBB, SMF, vB etc. afterwards, PB just seemed like it desires to be dormant in the stone age.
I think it's absolutely perfect for new administrators or for those who wish to have a look around a forum etc. but apart from that I don't see its position in today's competitive market. I would prefer this -
http://www.enochutah.net/AdmImg/Probs/ACP-1.jpg - to PB now. I do await what features they bring to the table and if they completely redo certain aspects of the software, but for five years it has remained the same, all they have changed (from a distant POV) is the style of their support forum (which you can only see once logged in).
Side note: I like it how they encourage multiple account creation for the same forum as a feature on their iPhone app. Most (if not all) forums that I've come across and have used have never allowed such account usage. Not sure why they'd use that as a selling point.