Someone's done a really nice home usability test on Ubuntu 8.04, using his girlfriend as the experiment. Apart from the good old-fashioned flame-bait value of this, I'm finding usability studies increasingly fascinating. From my own experience, it's remarkably common to find features that seem an obvious good idea from one point of view can be intimidatingly bewildering from another (and often I'm the bewildered one).
For example, I have one website that allows anonymous users to post content, although for obvious reasons each post has to be approved by an administrator. When content is submitted, the user is redirected to the site's front page, and gets a message in a little box with a different background colour to the rest of the page, telling them that their post is awaiting approval. Clear enough, you may think. However I got some feedback today from a user saying that the site is broken, because every time they try to post anything, all they get is an error message. You might say that the user should at least stop to read the message, but on the other hand something is wrong from a usability point of view if a message telling the user that everything is working perfectly fine looks at first glance like an error message. Usability is hard.