More from http://archives.neohapsis.com/archives/sf/www-mobile/2002-q1/0202.html
If the current machine does not have a network interface card, a statistically unique value is synthesized and CoCreateGuid returns a distinguished HRESULT indicating that the value is only statistically unique globally but is truly uniquewhen used only on the machine.
That should answer the question of uniqueness.
If the issue is randomness, it appears that they are generated in a fairly sequential manner, and so they should not be used in an application where randomness is necessary for security purposes (e.g. cryptography).