I work for a small non-profit radio station where we've got a couple of Ubuntu boxes (running 10.10, or whatever the latest version with standard GNOME 2 is). I've only done it because we recently expanded our volunteer base and didn't have XP licences to cover computers for every desk!
I've actually been really surprised at how well the volunteers have taken to using the machines, and how some even seem to prefer them to the tried-and-tested XP installation we've got on other computers in the building.
It helped that we were already using Chrome, Thunderbird and OpenOffice as our standard applications throughout the station - so when I installed the machines, I just stuck big, bold icons for each one on the desktop to make it obvious what to do. Audacity was a new one (we're using Adobe Audition on other machines) but people took to it reasonably well.
Comments I've received have been along the lines of "There's a lot less shit on the desktop than on those other [XP] computers," "It's easier to find what you're doing, and it seems a bit faster too," and so on. For some reason, our XP machines accumulate junk on the desktop, whereas the Ubuntu boxes stay relatively clean!
It works for a lot of our older, less experienced members who find it difficult to find, say, the Thunderbird icon among a pile of old documents and folders on the Windows desktop, or get confused when Start -> Email opens up an empty install of Outlook Express for some reason. It often feels like I spend more time helping people out when something's disappeared, they can't print, they can't find something on a Windows box than actually doing my job, so the Ubuntu machines have been a boost - they seem to just keep trooping on.
Of course, our users are simply booting the machine and opening up some standard applications that also run on Windows, and not going in and changing settings, or anything - I suspect that's where it would fall down in usability.
However, I've tried out recent pre-releases of Unity and GNOME 3 and found them pretty confusing, and I expect a lot of the users who've made positive comments to me about Ubuntu at work would too. It seems like a step backwards to me, with too many unclear mysterious icons, and bits of the UI whizzing on and off the screen while I try to work - and I'm sticking with the LTS at home, and not updating the work machines, either.