I've worked with Ubuntu 8.04 Hardy Heron for a little over a day, just enough to give a quick review of what's to like and what could be done better.
On the plus side, it's a generally smooth transition from the previous versions of Ubuntu. Nothing significant has really changed, and where there have been changes (e.g., the CD now gives a direct installation option, the default wallpaper is much nicer, and new applications for Torrent client and CD burning) they've largely been for the better.
On the down side are things that I expected to work more seamlessly but didn't.
A major disappointment is Firefox 3.0. Some plugins will work, but many others won't. This is because Firefox 3.0 is still so new and still in Beta and many plugins were written for the previous version. This situation should ease over eventually, but if you rely on several Firefox plugins to do your work, you may want to hold off on that upgrade until Firefox 3.0 becomes more mainstream.
Other disappointments stem from inflated expectations based on the announcements. I was expecting to see Inkscape as part of the CD, but it wasn't. I was also expecting to see some visible control for the new firewall but that, too, is absent.
The biggest disappointment, however, is the virtualization capabilities. I've got it working now but the experience was not as well-integrated as I thought it would be. There's still a lot of trial and error involved in getting it up (as opposed to VMWare before) and things like networking unexpectedly broke after I modified the settings as per instructions.
Final verdict: if you're willing to play with its new features, give Hardy Heron a try. But if your work demands something more stable, you might be better off with Gutsy Gibbon in the meantime.