Content Consumer has a usability review up of the latest Ubuntu 8.04 release, Hardy Heron. Using twelve tasks, ranging from the everyday to the less common, the review points out several of the problems Linux still has with reaching the mainstream computer user.
The author makes an excellent point about the trouble Linux runs into when attempting to make inroads onto the desktop:
The main issue with the desktop experience is that the geeky programmers and designers assume too much from the average user.
You can read the review here: The Great Ubuntu-Girlfriend Experiment.