[[Apple]] did what Linux failed to do. Delivered a usable and popular desktop operating system based on Unix.

OS X (Mac Operating system V10) replaced the Mac Classic operating system in the first of a couple of transitioning moves (the next MAJOR step was moving to Intel). It was brave, and it worked. The monolithic OS 9 was totally replaced, in every aspect. The core bits, memory management, disk management, IO, etc. were replaced by BSD Unix. The GUI was replaced with bits NeXt OS, and Apple moved everything forward in leaps and bounds while still offering a transition period for older software (which they didn’t do for Apple II software, to their great cost).

Leopard is OS 10.5 and all the pain of the OS X and Intel migrations is now behind Apple, and has proven to be the right decision.