I've upgraded to Kubuntu Hardy Heron, and so far I'm very impressed. I decided to stick with the KDE3 version in the end (mainly because Katapult isn't in the KDE4 version and I'm so used to it, and yes I know it's easy to download and install it), and it's working out well for me. Now that I've got /home on a separate partition, I didn't have to spend ages loading old e-mails or entering passwords for websites I used.
However, I'm using the Firefox 3 beta as my browser and I have one or two issues with it. First of all, a couple of extensions that I rely on aren't yet available (Google Toolbar, Google Browser Sync). Second, it's a real pain sometimes when I click on a link - sometimes it will open something random, like it will try to download something if I right click, rather than coming up with the right-click menu! Not sure why this is, but it's a shame as Firefox 3 solves many of the issues I'd had with Firefox 2 and is a LOT faster.
I'm also impressed with Xubuntu Hardy - great default wallpaper. I was actually seriously tempted by it this time, as despite my preference for KDE, I also really like Xfce (as well as IceWM and E17).
One very handy feature in Hardy is uFW, an easy-to-use command-line application which enables you to configure your firewall. Also, Kubuntu has better Compiz support.
All in all, while I'm sad to see Gutsy go, as it was so much more reliable than Feisty, Hardy is a worthy successor. It appears to no longer have the issue with DNS that I experienced with Gutsy, and the wireless works out of the box as it did in Gutsy.
What kind of experience have you had with Hardy so far? If you're using Kubuntu, have you gone for KDE3 or 4? Have you switched desktops? Or has Hardy given you problems?