Depends on what you want out of your UI. I ran Linux for years and years on my laptops. But getting a consistent look was difficult: Firefox, OpenOffice (at the time), Emacs, Inkscape, Gimp, the KDE CD burner I used, etc. all had different toolkits so theming never really looked right. Every time I upgraded Ubuntu something else broke: first sleep, then wifi, then audio (pulseaudio and jack, grrr). Getting on the network after an install was usually a bit of work. Maybe DHCP worked fine, ... and maybe it didn't. Configuring X (resolution, mousewheel, etc.) was a chore, although I think that's no longer necessary. I never did get my USB thumb drives to automount, although I heard tell it was possible.

That long, twisty maze of interlocking decisions, and the fact that every decision could and did go wrong, usually necessitating several hours of troubleshooting, that's bad UX.

Don't get me wrong, I love the idea of Linux. But Mac OS X is the real Unix on the Desktop.