I have a Mac OS X laptop and the X support is really not what it is cracked up to be. I can run remote X11 things OK but cut and paste is iffy and the experience somewhat suboptimal.
The killer failure of me is that I can *not* run mac os X application remotely, so I can't that M$ office as part of my desktop environment (which has a much bigger display). Unless you use relatively rare X extensions, most obviously xshm. that does not apply if you use X11 applications.
I am not arguing for the same protocol but I anything that does not allow me to mix normal applications running on multiple systems in a single desktop is several steps backward. Mac OS X fails the "can I run local applications on a remote display?" test. I hope wayland will not.