OS-X is a very mature version of the Mach kernel running under BSD. How similar is any Unix to Linux? The biggest differences appear to be placement of files, and that Linux, being Open Source, far more capable of utilizing wide ranging hardware and software.
I don't find the need to run any X-based programs, they appear kludgy and most of them exhibit the famous "one button, one function" of the X interface. That's just my opinon and though I find the OS-X GUI to be overly embellished it is as solid as a freaking rock under the hood. Solid VM subsystem.
In many ways it is a boutique Operating System, geared toward the advanced home digital market. In that sense it is clearly superior, in its proprietary way.
I am fond of it for many things and rely upon other Operatng Systems for other things.
patrick.