Our forum made heavy use of vBulletin's Joinable Usergroups functionality. Jon Wainwright (Waindigo) wrote an add-on for us that added this to xenForo and also imported all of the existing joinable groups data.
We also had several hundred "Paid Subscriptions" (Account Upgrades in xF) that needed imported. Jon wrote an add-on for this as well, so we didn't have to enter that data manually.
We also had a few thousand photos in vB's user albums. We're using the [xfr] User Albums add-on and its associated importer for this functionality.
We're also making use of Jaxel's excellent xenAtendo add-on to help publicize and organize upcoming events.
xenForo Enhanced Search allows us single-character search queries and 1000 result sets.
Syndol's Conversation Manager and Darkimmortal's Minorin editor toolbar added MUCH needed functionality... especially the editor toolbar. I didn't realize how many of our users were visiting on their phones and iPads and were unable to use the default WYSIWYG editor. We also installed Forum Runner and Tapatalk, but lots of folks prefer to use the default browser.
All in all, the migration went very well. Most of the user feedback has been legitimate usability questions relating to moved/renamed/etc functionality vs the old software. We also still have some work to do on the templates to make things easier to find and use (also a few spurious CSS issues, too).
From an admin perspective, we did (what I thought) was a lot of testing beforehand. The last few days have unearthed all sorts of great stuff that we never ran across during testing. The reported posts features are awesome. We're just beginning to figure out how to make good use of the warnings system. Alerts are great (but I knew that from visiting here). Oh, and the private conversations are SO much better than anything we had before. I was having a conversation with another user when I realized part-way through that another admin needed to be in on the loop. Added her in, and she immediately had access to the whole conversation thread.
Anyway, I figured that a (relatively) quick rundown of our process and experience would be useful to other folks. Feel free to ask any questions.