I just upgraded our company forums to VB 3.5.2 from 3.0.7. I can only state that it was easy and that I have noticed one huge advantage.

The forum itself loads almost instantaniously and the response from 3.5.2 is faster in regards to replies and such. With the old forum it loaded slightly slower. This is an advantage without a doubt that may overcome doubts (the faster the page loads, the more likely the client will view it and browse around after all). Also uShop is under development for 3.5.x series along with uCash. Almost all of the hacks for the 3.x series seem to be converting over especially with the holidays coming (more free time for people perhaps?)

Played around with some plugins and all I can say is...the plugin system will make administration easier and hack upgrades a breeze .