It is Unix with no driver problems. It is unix with both all free software and all proprietary software (except games). It is unix with a consistent pretty unobtrusive desktop environment that is easy to use and has sane defaults. It is unix with an accelerated window compositor that's always responsive and has no bugs.

Anyway, that was what it was in 10.6. Nowadays Ubuntu does all those things too to a certain extent, and it works without painful maintenance on a desktop machine. (I ran OSX on my desktop for a few years, it was very painful to upgrade, and apparently not in accordance with the EULA)

Still a great laptop OS though. And it runs rather well on macbook pros, which are the superior hardware choice in the laptop market, in my opinion.