Exchange 2000 vs competition

What would be the options for a company running MS Exchange 5.5 when thinking of upgrade? MS-Exchange 2000 is not really an upgraded version of MS-Exchange 5.5, it is a completely new product requiring Active Directory. AD deployment being a very expensive project, the question may rise: Should I go to Exchange 2000 which seems the obvious option) or any other competitve messaging application? Who are the players? Lotus Domino, Novell GroupWise, etc?
How does MS Exchange 2000 compare to the competition?
If you have done such a move (from MS-Exchange 5.5 to competitive solution), your feedback is greatly appreciated.

Thanks