... and if you have enough control over your environment to replace Windows Server as a domain controller, why would you be using the AD stack to begin with?
Aye, if one has that much motivation to chuck Windows for AD, then it's only logical that Windows on the desktop, Exchange, etc., are not on the scene.
I've done a lot of work with other directory servers too, and AD does the best job of any when it comes to multimaster replication and a few other things. However, using it purely for LDAP for an environment full of Linux and OS X machines is a tough call...
Why? I have such motivation, yet I have no intent on eliminating Windows from our desktops. That would be very disruptive to our 2.5K users, and Windows does a pretty decent job there. However, AD is very pricey due to CALs, and what it gives me is IMO not worth the price if I can replace it with Samba 4. That said, we're only beginning to explore this option, so it may not be viable or wise.
Just like many existing development companies have Windows-based infrastructure with both Windows and Linux on end-user systems, it makes sense to have Linux-based infrastructure while still supporting Windows end-user systems.
Linux support for AD also allows incremental migration of this infrastructure.
