When I upgraded the desktop and laptop machines, I didn't run into many issues. Had to reinstall the Nvidia drivers on the game machine. And deal with glances, but I mentioned that before.
I'll probably update the server to 13 next week.
I had to rewrite part of my php-based photo management system, as it relied on a library that was originally written for php 4 or 5 and hasn't been updated. Fortunately that wasn't too hard. There might be other things lurking like that, but they aren't critical problems - I can deal with them at leisure.