We implemented Apple Open Directory, serving ~400 users, using
four Xserves and and two Xserve RAID's. We're using Apple's
mail services, file, web, web log, and VPN service.
So far, things have gone better than I expected. We are
authenticating Mac, Windows and Linux PC's, all of which can
access the same home directory. The Open Directory master server
also acts as the Windows PDC and serves up roaming profiles for
Win XP clients.
What I've been hounding my Apple rep about is the lack of a real group callaboration suite. The pieces are there; iCal, Address Book, Jabber, Cyrus/Postfix. They need to be brought together in an Exchange/GroupWise sort of fashion. We are still using Steltor Corporate Time (now Oracle Collaboration Suite) for calendaring, task lists, and shared contact lists.
I'm watching the Hula project closely. Rumor has it Apple is shopping around for a comprehensive group collaboration system. Hula might be it!