This. It's almost as if Linux is a collection of parts and tools from which a sufficiently good designer can build a usable desktop whereas OS X is a usable desktop. That's an exaggeration, but for me there is some truth to it.
Note that Linux was my only desktop from 1997 through 2009.
This has been highly successful, only I'm afraid that the Unix people inside Apple are loosing influence. The sandboxing in ML looks like a mess to me. Not that its a bad idea per se, but look what it did to our beautiful '~/Library/[ApplicationSupport|Preferences]'! It's tacked on and it's obvious that this has been a political decision from management rather than engineering. Makes me angry.
On a different note: I've never really used it extensively because I switched to macs in '05, but wasn't Ubuntu pretty close before they switched to Unity? I remember last time I installed it that for the first time I could have a Linux that worked out of the box, including network and graphics drivers.
Which is fine, it's Apple's decision to take, but it's no longer for me.
(As for Ubuntu - I'm using pretty standard hardware and never had a problem)
Anyway, this has been almost 100% true up from 10.3 through 10.6.x. Recent developments both on Apple's as well as on MS's side have closed the gap between the two systems quite a bit. I still prefer OSX but it's gotten close. The real reason I continue to buy macs nowadays is the hardware quality.