To be quite honest I don't see all the fuss over VBulletin 3.5.0. Sure it uses AJAX in a big and well documented way however the templating system and modification system for a currently existing forum will be altered considerabley.

If it's a new forum starting up, great, however unless you're willing to pay the price and spend the time sorting it all out, then it will be worth it. Otherwise, if it's not broken don't attempt to fix it