I'm a veteran Linux user but have moved to OSX some time ago, since it gives me the UNIX I need, and the GUI I so sorely crave.
BUT recently, I was trying to get someone's computer up and running, and Linux was the only thing that would install due to some bug or other, so I temporarily put an Ubuntu install on their computer. Decided it would be a nice experiment for a non power user, to see how well they could cope.
He hated it. He couldn't get flash going, so it wouldn't work with certain sites. He was having trouble doing basic navigation of the OS, and had no idea which programs really did what beyond the basic.
There were a host of other issues I can't really remember now, but it was a very frustrating experience for him, and he was very happy when he got his Windows 7 back.
I sat him down with my macbook and he seemed to figure out OSX handily.
The Kernel works well. The OS handles many things very well internally, but the overall user experience, while MUCH MUCH improved over how things used to be, just is not as easy to use as a Mac or Windows computer.
The real work needs to be done by UI designers with coders to support them. Even connecting to a wireless network can be a chore. God forbid a driver doesn't work or something along those lines and you need to open a terminal.
While you'd think the 'many eyeballs' thing would take care of something like that, it seems all these eyeballs and the heads behind them just want their OS to work, and for a non power user right now, I wouldn't call it ready.