I have nothing but good things to say about Xenforo.
We migrated from VB to XF this past Sunday and according to Google Analytics, the following Monday and Tues has received the most traffic we have on record the past 2 years.
We have a custom XF mod that track status of college applications and after a day, data in that mod is already found by Google, some keywords rank on top 5, higher than the official websites of that specific program.
The pageviews, time online are up. Bounce rate is down.
I'll post a detailed report after a month to gather enough data and to eliminate biased sampling due to people new to the design.