I have used invision in the past, though version 3 I believe. and suffered catastrophic database failure every few weeks with it. We used it very briefly over at www.counter0strike.net and it was horribly unstable and slow.

I think part of the problem stemmed from the lead programmer leaving to set up Ikonboard, an almost identical bbs system.

vBulletin we also used at counter-strike.net and it was okay, just got very slow with over 100k users! We probably used Vb for the longest of all but the current system. It was the one we had the least problem with hackers on too.