[M]eaning that Leopard conforms to the Single UNIX Specification Version 3 (SUS), which defines how components like the command shell, the compiler, and C APIs should function and behave.
Now I have mixed feelings about Mac OS X. I use Windows Vista myself of course, but admitted, the OS X UI looks kind of cool. That is not surprising because the UI is the proprietary part of the operating system. The underlying Unix was originally based on FreeBSD as I understand it. FreeBSD is under a BSD license, also open source, but not as bad as the viral GPL.
So I guess that if you have to use a Unix system OS X is a reasonable choice, better than Linux and Solaris anyway. At least Apple has people who are paid to work on this OS.