One of the most important issues facing Vbulletin forum owners thinking about switching to IPB is the reaction from the users. If the user experience changes too much, we could really upset our user base.
I am happy to report that since going live with my IPB conversion yesterday, I have had a lot of good feedback on the switch. However, there is one feature that many of my users, and myself, are missing from Vbulletin: in Vbulletin when a user visits the site, all threads that have new posts since the users last visit are highlighted in bold in the forum view. This is a great feature and my users really liked it. I'm hoping that IPB will consider implementing this to ease the transition from Vbulletin to IPB.