>>10
No more than BSD is. All the good OS X has is thanks to their use of the Mach kernel, which is a good kernel implementing an UNIX OS.
Windows itself is great in its stability of its API, ABI, driver interface and other binary interfaces. This lets you write applications and drivers which will run well on a variety of NTs and even in 9x if newer APIs aren't used. I don't see Linux ever achieving such solid binary compatibility, given how much kernel and usermode interfaces change.
(I obviously don't care about GUIs, so don't mention that to me - be it Windows' or OS X's).