Just before Christmas I wrote a post about upgrading our development and test APEX installations to the new 5.1 release. We’ve now got it in all our production environments too. 🙂
As far as I’m aware we only had a single issue, which was not directly related to APEX and was actually my fault. I forgot to add the APEX_050100 user to an ACL, so some non-APEX code that was doing web service callouts using the APEX_WEB_SERVICE package stopped working. Once that was added to the ACL all was good.
As always I must stress our company is not a massive user of APEX and most of our applications are fairly vanilla APEX against 220.127.116.11 and 18.104.22.168 databases, so I’m not suggesting your experience will be the same as ours. That said, the move from APEX 5.0.4 to APEX 5.1 was really painless for us.