So far I've set up several small offices using Samba4 as a drop in replacement for Active Directory. Here is what I've found it does well: Windows Authentication, AD DNS, Group Policy, Easy scripting (python tools and libraries). What it doesn't do well yet: Replicating AD with other servers. I haven't had much experience using subdomains, etc, mainly because I haven't been able to get it to replicate. But for a small office, it works fine.