We started out as youthdebate.co.uk but soon decided to move to an international domain and allow registrations from elsewhere. Our forum software has been myBB 1.6 since our inception. There were always a couple of little things that bugged me about the 1.6 edition of myBB. I read it wasn't the most secure, either. It's old software. Still, myBB had a lot of great points to it, and it was free, so I have nothing but praise for the myBB team. I think we just outgrew it though. I was getting too many bug reports.
Anyway, when myBB 1.8 was released we figured we'd update, but then on realising all of our plugins would become incompatible, we just thought we might as well take the plunge and seriously upgrade our forum software. We chose XF, and I'm extremely glad we did so now.
Migrating was a doddle, the in-built tool was really great. I had to refresh it a couple of times when it got stuck, but no biggie. We lost all of our likes (only available with plugins in myBB), but it could have been worse. I'm sure there was an easy way to save them, but I'm not confident messing with the database. I'm one of those awful admins you all hate who doesn't really know anything about being an admin, and just kind of wings it and somehow gets by.
Our theme is now much sleeker and modern looking (we're using 'Modest'), the site is far less buggy, navigation is much smoother, alerts are better, PMs/conversations are way better, and I'm loving the sidebar - using the widget framework add-on.
Our users absolutely love the move, and I think if I ever moved back they might just kill me.
But to what I deem to be the most important thing from the move: SEO. Our SEO on myBB was never great. We used a plugin which never really worked right, and we got just a few hits per day from search engines. It was about 2% of our traffic. Shocking, actually. Since the move, organic traffic now makes up about 25% of our new traffic, and 12% of traffic including existing users. It's still low but the percentage is growing daily. I can't thank XF enough for that.
But yes, the move was great. If anyone has any questions I'm all ears (or eyes, rather)