Our forum has more than 3 Million posts and we used the Xenforo built-in importer because we had problems with the big board importer add-on. The downtime was about 15 hours from closing the old forum to opening the new one. Tweaking and rebuilding the caches was done after reopening the site for our members.
The community reaction to the new system varied from very welcoming to refusing the change but after few days of using the software, and by our administrators' posts about the new features, everybody seems to welcome the new change.
For the style, we used Xenfocus' style (Novus) which we preferred because of the gray posts' background. to their designers.
We are using the Resources Manager as a Bookstore with Arabic translation made by @bousaid