Office Importer fails to start LibreOffice 3.5 in server mode (I tried the internally managed server configuration only). OpenOffice.org 2.2 and Apache OpenOffice 3.4 worked fine.
The lowest-level error is "org.artofsolving.jodconverter.office.OfficeException: office process died with exit code -1073741515". The last piece of XWiki's code on the stacktrace is "org.xwiki.officeimporter.internal.openoffice.DefaultOpenOfficeManager.start(DefaultOpenOfficeManager.java:183)", from xwiki-platform-office-importer-3.5.jar.
This issue, if still present in XWiki 4.2 (I didn't test), makes Office Importer incompatible with LibreOffice at all, because versions of LO prior to 3.5 are not available for download anymore, and new versions are being released at a very fast pace. The documentation on http://extensions.xwiki.org/xwiki/bin/view/Extension/Office+Importer+Application#HHowtouse mentions LO 3.4, being therefore outdated.