So far my Hardy experience has been good. I've had a few crashes with Firefox 3 but it's session recovery works well so it's been only a minor annoyance. I have mixed feelings about the new live searching in the address bar as I'm used to it only picking up URLs as in Firefox 2. When it hits on page titles and other data it's distracting.
Some of the bugs I encountered in alpha 5 have been fixed. SCIM can now be disabled via System > Administration > Language Support > Enable support to enter complex characters (unchecked). The Gnome display properties applet and Gnome settings daemon no longer crash.
The Java plug-in problem may have been my fault. The correct package is sun-java6-plugin and I may have installed a different one by mistake or it wasn't in the repo at the time.
The problem with X freezing the system during a VT switch or restart/shutdown is still present. According to bug #204603 it's an Intel driver issue which requires Option ForceEnablePipeA "true" to be specified in the xorg.conf Device section. This may be a repeat of a previous bug.
The battery monitor bug only occurs with one account and only with the Power Manager applet. The Battery Charge Monitor applet shows the correct state and other in other accounts on the laptop they both work correctly. Minor config problem somewhere.
A major usability problem I've noticed is that the drive mounting/browsing options that were in the "Storage" tab in "Removable Drives and Media" has been moved to Nautilus preferences. The menu entry hasn't been renamed however which is confusing because it no longer affects removable media. Bug #210499 was already filed about it so I'm not the only one who noticed.
One glaring omission in new Hardy features is the lack of networking support in Friendly Recovery - the menu you now encounter after selecting recovery mode from the Grub menu. Unless your system is configured to use static IP/gateway/DNS you don't get any network access so apt-get is rather limited. On a DHCP network like most broadband and wireless connections you have to manually start dhclient first.