I just read through the 15 pages of this thread, and look forward to the conversion. I used to own two large discussion boards that I converted from vB 3.x to XF 1.5. As with some of the comments in this thread, many threatened to leave if anything changed, and some did. Most remained.
One active member (over 25,000 posts) reacted very negatively to the conversion because he couldn't access the site with Tapatalk. When I explained to him that XenForo works with all mobile platforms and outperforms Tapatalk, he wouldn't accept this and demanded that I delete his account. After a few back and forth exchanges, he again demanded that I delete his account. I told him I would be happy to purge his username from the website, but would not delete the content since this would be too disruptive to the flow of the years of contribution. I also cautioned him that this would not be reversible. Again, he insisted, so I made him disappear. About a month later, he used the "Contact us" feature to request his old account back. He eventually accessed the site on his phone, and was pleased at how it looked. Too bad, so sad, but no.
In my opinion, vB 3 was the last stable version with regards to security and data integrity. Unfortunately, it was no longer supported and the wolves were getting smarter. The vB 3 developers left to start XenForo, and the attention to security was immediately obvious.
One thing to consider as a user, is to export any important PMs prior to the conversion. XenForo treats these differently than vBulletin, and I don't know of any way to preserve a PM thread after the conversion. XenForo refers to private messages as "Conversations". These are similar to private threads instead of email-like exchanges. There is no Inbox or Outbox in XenForo, just private conversation threads.