I think the problem is that this attribute is empty for a lot of objects but you can verify this by using another attribute.
If you want to get results quickly it would be better to use IDirectorySearch since the interface you are using now is really meant for scripting languages and is inherently slow.
BTW: the offer that you can test the new Active Directory
unit for
Jwscl which nicely wraps AD in classes and makes it accessable the Delphi
oop way.