This is mostly a stabilization release leading to XWiki 5.3 final and it brings a few small improvements to the Solr search UI and the ability to import large XARs using the WikiStream framework. The Solr index has been modified to support the search for documents matching specific values in multiple object properties. As a consequence you can now facet and sort the search results based on property values. Besides this the new release includes 38 bug fixes and 14 small improvements.
Full release notes available at http://www.xwiki.org/xwiki/bin/view/ReleaseNotes/ReleaseNotesXWiki53RC1
See http://jira.xwiki.org/secure/IssueNavigator.jspa?reset=true&jqlQuery=category+in+%28%22Top+Level+Projects%22%29+and+fixVersion+in+%28%22%35.%33-rc-%31%22%29+and+resolution+in+%28%22Fixed%22%29 for more details