If your users are experiencing issues with their Outlook Calendar, Microsoft released back in May a tool to help administrators in these situations, the Calendar Checking Tool for Outlook (also known as CalCheck).
This is a command-line tool that opens an Outlook profile on the local machine, opens the Outlook Calendar and checks permissions, free/busy information and auto booking, for example. The tool then checks each item in the calendar for any problems.
The calendar to by analyzed must reside on an Exchange Server and this tool does not work with IMAP, POP3 or any other non-Exchange mail servers.
To download it go to this link.