We currently have a two-node database availability group (DAG) protecting our mailbox databases. Over the weekend, a person or persons unknown shut down the physical server hosting one of the DAG members. No one, including me, noticed any difference— all our users continued to work normally.
Failover was completely seamless, and neither our Outlook nor OWA nor mobile users contacted me to complain. I only became aware of the problem when I was troubleshooting our back pressure incident.
Exchange 2010 rocks!