In what type or roles and network is Mac OS X server commonly used. What does OSX server have that is not available on other Unix enviroments like Linux, or OpenSolaris. What benefits does it have over the Microsoft?
One place that OSX server is pretty common is in K12 schools. The combination of Open Directory, Workgroup Manager, and other features make it a nice server for a small network that has mostly OSX/Linux Desktops. Typically OSX server will be used to provide most of the common services like fileservering, web, mail, dns, dhcp, and so on.
The advantage OSX server has over something like Windows Server is that Apple doesn't try to nickle-and-dime you to death for buying Client Access Licenses. You can cheaply buy a OSX server license that allows an unlimited number of clients to connect. You don't have to pay per user for access, and pay another license per user for mail server access.
Also check out this facility devoted to Mac Mini Colocation.
I setup an Xserve not long ago at a company that was all Mac-based. It provides a calendar server that all the Macs could easily use, as well as easy to setup and admin file sharing and centralized time machine backups.
Most of the places using OSX server soft and hardware I've seen seem to be Mac-centric.