I migrated from vBulletin 4.1.x (if i remember correctly) and the migration was totally painless. I used the 3rd party tool created by @Jack Bunce. Kudo to jack for the great work. Starting from 1.2 version of XenForo now you have an official importer.
I installed also the wonderful Post Ratings add-on by @Luke Foreman . To be honest, this was the feature i saw on another forum that grabbed my attention on XenForo. I don't know if this could be hard from a commercial and legal point of view but @Kier @Mike @Ashley , please, consider transform it in a core feature finding an agreement with Luke. That plugin extends in the right direction the sense of the boring facebook "like".
Also, consider extend the official smiley set with @Shelley one.
I'm not into php developing so i can't judge the quality of the code of Xenforo but i can express my feedback about the admin and user experience.
From an admin point of view every things are much more "quick". Xenforo is the right forum for "lazy" admin and moderator. The remove spam tool is wonderful. Permission system could be a little pain to understand at the beginning but after reading official doc and @Brogan guide you will manage it and you will find it a very powerful and EASY system.
By the way, please add a feature to move message to another thread without needing to create a "temp" thread!.
The same from an user point of view. A ton of "little" features that for sure are not "rocket science" but that improve a lot user experience: notification system, tagging, great wysiwyg editor and attachments management, thread preview, improved private message system, etc.
Generally speaking one of the thing that i like a lot is the "smart" use of ajax stuff. What i perceive is that @Kier and @Mike do not put tons of unusefull AJAX stuff just to obtain a "WOW!" effect on user. They use it only to improve user experience. All what a user need is at hand. No need to click on a lot of links, navigate a lot of pages, waiting a "lot" of time before make something. All is studied to make the forum utilization a pleasure.
Ninja Edit: oh! i forgot to say that support here is at top. Well to be honest i never had the need to open an "official" ticket to support. The few things i needed to know were asked here in the forum and the community itself is a kind of 1st level support with knowledge comparable to a 3rd level one
PS: i have written this thread here because i see this is the "feedback" forum. If this is the wrong forum feel free to move it on another room.