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.
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...