First, everything went smoothly. Not one real problem. Ok, there seems to be something weird with marking threads unread (some very old threads were marked unread for some users), but I don't consider this to be a real problem.
The next step was to customize some of the layout and stuff like that. I found it (with kind help from Brogan and some others) amazing easy to customize anything. The strict use of CSS and the template system makes it real easy to do what you want (let's wait for the next update...).
The only thing negative about xf is, that there are some features still missing (from my point of view), but I don't regret the change. Users like xf, mods like xf, it feels way faster and even Google likes xf.
So if you are thinking about a change: don't hesitate, just do it. It works, you most likely won't regret!