I recently upgrade my Gusty laptop to Hardy and it fixed a very annoying video bug that I'd been having. So far I'm quite impressed by it. I haven't yet run into any of the issues that others have been complaining about.
I upgraded, and a few annoying problems (the biggest being lack of hibernate) were fixed for me.
What I wasn't thrilled with were the compulsory upgrades of a number of apps; I had to remove firefox3 and install ff2 (don't care for some of the changes), lost XMMS, and KOrganizer has a less-simple UI for managing reminders, among other things.
(Luckily I was not compelled to jump to KDE4. :) )
