We went from Classic to Threads about 9 months ago, and couldn't be happier. Back then there were some things that users missed, like the "quote" option. But newer versions (like the one you'll be getting) have gone back to that welcome feature. The other thing that confuses "migrants" is that now they must reply to an individual post, not just the thread itself. But they'll get used to it.
In terms of concurrent users, we had ours set to 110. We bumped into that limit this week, and raised it again. The limits will be imposed by your mySQL settings, for instance, and not UBB Threads itself.
Running with 120 users concurrently it's still lightening fast.
http://www.bmwrt.com/ubbthreads/ubbthreads.php
