A few days ago a customer asked for changing permissions to show only free/busy information for some conferencing rooms. To meet this requirement I used the following PowerShell command:
Set-MailboxFolderPermission -identity ressourcemailbox:\calendar -accessrights availabilityonly -user default
If configured like this, a default user can only see free/busy information for the room ressource. Attendees see the full description in their personal calendar.