...is that the users love it. Outlook sucks for email but it's not bad at managing contacts and diaries.
Public folders, shared calendaring, shared contact lists - it's all fab at the client end.
As I know and you know and Andrew is discovering, at the other end it's a complete bear. Has to run on Windows, 200x versions require AD, yadda yadda.
But there's nothing to touch it for a groupware solution. Not Groupwise, not Notes, not OGO. And that's A Problem.