I have been using Ubuntu Linux exclusively for nearly a year now, with one abortive attempt to return to Windows. For me, the transition has been smooth. But to date, I have not exposed anyone new to Ubuntu.
But recently, I hired a law clerk to help me with a few projects, so I bought a new desktop computer for her. I went with a Dell computer with Ubuntu Linux pre-installed, and when my law clerk showed up on day one, I just showed her how to log in, where to find my client folders on the network, and not much else.
After a week using Ubuntu, she has not come to me with any problems or asked me how to do anything. I am sure it helps that most of my practice management software is online, but she had no issues getting up to speed with OpenOffice.org, the Evince PDF viewer, or anything else.
She says it feels like a mix of Windows and OSX, which is pretty much what I think about the default Ubuntu interface.
So for those who might want to try Ubuntu, give it a shot. It is easy to get oriented, and it just works. Plus, you can download a LiveCD and try Ubuntu without installing it or affecting your hard drive.