Hey. I'm going through this right now. My server was 11, and I wanted to go to 13. I definitely didn't want to get into a situation where the server required hours and hours of repair.
I'm halfway there. The upgrade to 12 went smoothly. The biggest headache was glances, first from the lack of web interface (which I was ready for), and the lack of RAID support (not ready). I might do the switch to 13 next week.
Did you go straight there, or go through 12?