I've been running Hardy for about a month, and I've followed the upgrades pretty closely. I'm quite happy with it. A number of minor bugs were fixed with my ACPI, and theres numerous application upgrades that are nice (Firefox 3, Gnome, etc.). The new default theme is also more vibrant... not that you can't change them, but it's nice to see that kind of attention to detail.
IMHO, worth the upgrade.
