I installed Ubuntu Hardy Heron via Parallels today and I am once again amazed at how sophisticated Ubuntu has become in terms of usability and offering the "just works" Linux experience.
There are many new features including some new interface customizations which allow you to customize your experience. The release seems more stable that Gutsy Gibbon but that could just be my hardware.
I've been a proponent of the Linux desktop for five or so years and the thing that continues to stifle adoption is the lack of drivers from some major hardware vendors (which seems to be easing) and the lack of a well-rounded office suite. Somewhere there is another Mark Shuttleworth who will solve the applications problem and make life easier for a lot of people.
Lifehacker offers a great round-up of Hardy Heron's new features.