I think they all discovered there are different definitions on "usability" depending on the source, but generally usability is about real people doing real tasks in a reasonable amount of time. Some researchers also refer to "learnability" and "memorability" to define usability.
And as they found, usability is not the same thing as user experience. Usability is about getting something done; user experience is more about the user’s emotional impression. I believe that usability and user experience go hand-in-hand. Programs need to be pleasant (user experience) but people need to be able to use them, too (usability).
In a comment on Renata's blog post, I wrote about some edge cases in usability and user experience. Most programs that have good usability will also have good UX, and most programs that have poor UX also have poor usability. But that's not always the case, and I gave two examples: a game and a file manager.
I'll post the week 2 research topic very soon. As always, I first send this as an email to Renata, Diana, and Ciarrai, and make a copy of my emails as blog posts on my Open Source Software & Usability blog, so we can use the comments as a sort of discussion forum. If you follow my blog, feel free to participate and comment! Expect a new post every Sunday or Monday.