The Mac mini server was already a great machine. The latest version, the Mac mini with Lion Server, is even better.
The ninth major release of Apple’s server operating system is as big a change as the change from OS X Server 1.0 to OS X Server 10.0. In many ways, Mac OS X Lion Server (version 10.7) succeeds, but it is hampered by UI annoyances and inconsistencies that will probably be fixed in future updates. But right now, using Lion Server is a tad more maddening than it should be.