But what are peoples thoughts and/or experiences with VSS quiescing for applications like SQL, Exchange and AD? Has the replicated data been consistent when recovered at the DR site? Do the database writers get flushed correctly/as expected?
My experience has been that SQL 2005 SP2 and later on Windows 2003 and later is usually consistent. Usually is the key word though, in that is important to test the replicas for consistency. I can't speack to Exchange, as I have never replicated it this way. AD will depend entirely on the version of AD in use. vSphere 5.0 U2 and 5.1 support VM-Generation ID, which allows Windows 2012 DCs to be replicated this way and be supported. Check out http://blogs.vmware.com/apps/2013/01/windows-server-2012-vm-generation-id-support-in-vsphere.html for more info on this.
I'm not 100% certain on the SQL support from MS with this scenario either - I have heard conflicing reports and have not seen a definitive answer to this.
The approach I like to take with SQL (when possible) is to use replication, local .BAK files or agented backups and virutal machine backups that also get replicated to the DR site(s). This way you have multiple options to restore from, if one of them doesn't work for whatever reason. As mentioned earlier though, testing the replicas is crucial.