Conversion was mostly flawless, users seem to like it so far.
Only downside that causes lots of headaches is our CMS content. We had plenty of articles in vBulletin's CMS and while we've managed to port the content to a forum category, we're now struggeling with some nasty side effects.
1. Many threads from articles only contain a (invalid) link "read article" instead of the actual content.
2. Moving these threads (or should I say the first post of a thread) to the resource manager breaks embedded attachments. Lots of manual work required instead of a nice bulk action.
3. elasticsearch seems to have a problem with advanced search for threads (instead of posts). Only one and always the same thread appears in the search results. But for standard searches it's fantastic! 4 minutes to rebuild the search index of a 1.5 million posts forum, compared to 36 hours with vBulletin ... that's some serious improvement.
Anyway, it's not a simple task to move a big forum to a new platform and we're really happy it worked nicely.
Great experience so far!