Recently upgraded 2 production servers from 11.5 to 12. Both on CentOS 6.5 very latest updates.
Went quite well with only 2 minor issues after the upgrade.
Did read a lot on the forum to prepare. Backup of servers essential - just in case. Certificates have caused issues, make sure yours are ok. Packages of other repos apparenty caused the odd issue. Don't use other repos ourself but would advise to temporarily disable them, than re-enabe after upgrade.
Upgrade itself went very well.
Quick systems check after the upgrade showed that the clients websites weren't displayed. Logged into server console and
# /usr/local/psa/admin/bin/httpdmng --reconfigure-all
sorted it. [Had the same issue when upgrading from Plesk 11.0 to 11.5 a long while back]
Second issue was discovered a little while later. Compression of log files wasn't working as expected. As it doesn't really affect clients it was tackled the day after.
All-in-all quite smooth. Thanks to Plesk team for doing a very good job on the upgrade path!