We're still testing dirsync to ADD for O365, and in the few months of testing the sync has stopped twice. Once because of some organisational/Live ID confusion in Microsoft accounts and once because of a (presumed) expired password. In both cases, we only discovered that sync had stopped when something that was required to sync failed.
To be more pro-active, is it possible to configure an email alert when (for example) the time since last dirsync exceeds 4 hours, and repeated alert if the time since last dirsync exceeds 6 hours?