We just hit 1 year on Xen and thought I'd share some stats for those who like that stuff. We came from SMF 1.1 so I can say from an admin/backend experience its been worth it there alone. We had regular server resource issues with SMF that we have been able to avoid with Xen.
Now in terms of engagement with the actual users its been a huge success. In 1 year we hit 1 million likes after a slow start with the feature.
On top of that we've had about 2.5 million new posts in the year which is not a big change one way or the other from past years but any concern over losing activity was quickly alleviated.
It has been a good year and look forward to another good one, hopefully running on 1.2.