let me share my experience of the AADConnect synchronization tool (preview) … this product will replace DirSync/AADSync soon.
when you install and use AAD Connect tool, don’t be surprised if there is no user’s passwords synchronized to Azure AD !!!!
the source connector must use a valid source AD on-premise technical account with rights to read user’s password (be member of default administrators group)
add this technical account member of local ADSyncPassword group (empty by default!)
Restart the Azure AD connect synchronization service!
How to check if user’s passwords has been synchronized?
=> You must check if EVENT ID 657 exist on Application event log
=> test to login using the standard synchronized account using IE on the portal: http://manage.windowsazure.com.