There are very few niches/users left that actually want Windows > Apple, and most of them begin with "gam" and end with "ing".
But, users are pulling these organizations, kicking and screaming into a world in which everyone wants a MacBook Air/Pro.
I really don't envy Microsoft.
Open Directory is basically a ldap with MIT kerberos that can sort of integrate with AD. Most folks setup a hybrid (referred to as the "magic triangle") where you use AD for user auth and OD for computer policy stuff. It's not very robust, and Apple isn't exactly committed to the notion of server operating systems, so it doesn't get much love.
You really need to buy a client app like ADmit Mac, or a server-side solution like Centrify to really work with Active Directory.
AD itself is a really brilliant collection of software. Think of it as a fully integrated, smoothly operation bundle of LDAP, Kerberos and DNS -- it "just works".
Coming from a Unix background, AD was amazing to me as there isn't any Unix equivalent at all, except maybe for the ancient NIS and NFS integrations. You get single sign on. Robust Access Controls. The ability to trust the identities of external organizations. A policy engine to do all sorts of management activities at various levels. Oh, and it allows you to easily build fault-tolerant deployments without much forethought.
Now, you can do this with OD, but it's not as easy as "built into all windows installations, set it once and forget it".
The way Microsoft prices Windows, at least at retail, and the way they segment their OS features for different markets, are really annoying and somewhat painful.