I would appreciate feedback on experience with Confluence 4.0 from people who have implemented this version. In particular have you found the 4.0 release stable in your environments?
I've found it exceptional on my Ubuntu 10.10 - stable, but I'm running a limited number of users.
Installer was improved. I miss full-page wiki markup, but clipboard support to copy and paste images directly on a page is excellent.
Personally speaking, it was worth the upgrade.
I'm developing plugins for confluence. If you have a confluence 3.x instance with lots of 3rd party plugins - upgrading to confluence 4 won't work because most 3rd party plugins are not (yet) confluence 4 compatible. Check compatibility for each of the plugins on https://plugins.atlassian.com