OS X Server, Apple's first serious attempt at a server operating system, gets a fair amount of credibility out of the gate for being based on FreeBSD, an operating system of undisputed reliability, power, and obscurity. Apple's the perfect company to do something about BSD's ease of use problems.
As proprietary server operating systems go, OS X is not expensive, but it's not free by any means, and it runs only on Apple's hardware. Let's assume for the moment that Apple's hardware is high-quality and high-performance (although only testing could demonstrate that and I haven't done such tests). Much to my surprise, Apple's Xserve rack-mount servers are actually competitively priced. OS X Server OS itself is available through a three-year subscription service. For a large number of clients, the prices are cheap compared to Windows 2000 Server or Solaris, and there is an unlimited client license available.
In terms of software, OS X Server includes all the usual Unix suspects, such as Apache and Samba. Many large ISVs have committed to ship versions for it, or are already shipping, as is Sybase with its Adaptive Server Enterprise 12.5. But if it were just another BSD box then it still might not compare favorably to an x86 box running FreeBSD.
This is where Apple's strengths start to come through. OS X includes a number of GUI tools to ease the BSD administration process--a process so difficult that (hyperbole alert!) it makes Linux seem almost as easy as a Mac. There are several remote-service administration tools included so that you can monitor, start, and stop services from across the network (rather necessary for headless servers like the Xserves). It also offers GUI interfaces to standard server components like Samba and FTP, but there are still some cases where you must resort to the command line. Still, compared to out-of-the-box BSD, OS X is an ease-of-use windfall. Day-to-day administration is spent mostly in Apple GUI tools such as Workgroup Manager, rather than in emacs and a Kourne shell.
Aware that few enterprises will be running Apple-exclusive networks, Apple built OS X to integrate easily with existing networks and their directory services. All the standard applicaotin authentication services, such as Samba and Apache, are integrated with Apple's Open Directory (OD). OD can, in turn, cross-authenticate to many external LDAP directory services such as Microsoft's Active Directory and Novell's eDirectory (NDS to most of us). So it's even possible for an Apache authentication request on OS X Server to authenticate against eDirectory on Netware.
I have to say I'm favorably impressed with OS X Server at first glance. It's a really good first version. The directory integration features are impressive, but of course these are requirements for any new system that one would try to sell to enterprises with heterogeneous networks. In terms of making Unix accessible--assuming your administrators are willing to admit that it matters--I trust Apple far more than I trust Red Hat. GNU people like to call GPL software "free" software, but Apple's proprietary interest in OS X gives them the freedom to leverage the Mac interface and make Unix much easier to use--something the GNU crowd would never do.
The problem is, that as good as OS X Server may be, it's different from the Linux/Unix, Windows, Netware, and other server OSs enterprises have been buying all these years, so why should they consider it? I would definitely consider it as an option for applications that require Unix or Linux servers. But tread lightly; there's too little real-world experience with this product to recommend it blindly.