I upgraded my personal laptop to Hardy Heron last Thursday, along with everyone else. The download (just under 1GB for all the packages I needed) was surprisingly fast for what must have been an immensely busy time, and completed in less than two hours. I later heard stories of some people waiting several days for the download, so I guess I was lucky.
The most obvious difference (probably for almost any user, but particularly for a web developer) is the inclusion of the Firefox 3 beta. The first thing any developer will notice is the disappearance of Firebug, which will tell you that there is no Firefox 3 compatible version available. Don't panic! Manually uninstall Firebug, then go to Applications -> Add/Remove... and reinstall Firebug from there - it will work just fine with Firefox 3. Now, with that crisis averted, you can start playing around with the new features in Firefox 3. My personal favourite was the improved location bar autocomplete; I was also delighted with the GNOME integration. Jonnie was more impressed with the new bookmarking functionality. Of course, it's still a beta, and not everything works perfectly, but it's already a nice improvement - and if you need it, Firefox 2 is still there too.
Firefox aside, I didn't notice a whole lot of new functionality in the apps I use regularly. A number of severely irritating bugs have been fixed, however. The passphrase prompt on booting from an encrypted disk now recognizes the backspace key, so mistyping the last character of my passphrase no longer means pressing Enter, waiting, and starting over. Audacity once again works in full duplex mode, so I can play back and record audio at the same time. I have pretty much neglected to record any music at all for the past few months, thanks to this bug. The Last.fm client in universe has also been updated and single-track previews now work, so it is no longer necessary to use the external Last.fm repository.
The IcedTea Java 7 package has been replaced with an OpenJDK Java 6 package, and (more importantly) the wonderful jEdit text editor / mini-IDE has been included in the universe repository, and seems to work more or less flawlessly with this JRE. I highly recommend jEdit for any sort of development (even for Windows and OS X users - like most Java applications it runs equally well on all platforms), and I use it almost exclusively at work, switching occasionally to ViM for some very small tasks.
One caveat with OpenJDK is that if, like me, you had manually run update-alternatives to set all your Java CLI tools to IcedTea instead of GCJ in Gutsy, you will have to do the same again to switch them to OpenJDK in Hardy, as the installer seems to miss this step and leaves dangling symlinks pointing at the old IcedTea binaries.
The one thing that I did really dislike was PulseAudio. It just doesn't work very well at all - I got lots of stuttering with most applications - and when it does work, it hogs the sound device and prevents any non-PA applications from using it. I switched the defaults back to ALSA and I would suggest that anyone else having trouble with PulseAudio should try doing the same - at least for now. Maybe it will improve as time goes on, and then I will switch back.
Overall, while I am quite happy with Hardy, I'll be waiting a while to see how it settles down before I start using it at work, or recommending it to anyone else.
