I've used Unity on my desktop for about a week. As a UI designed the the netbook it's, of course, not ideal for regular desktop computers, but since I have only a single desktop I had to make do. It makes for a slightly frustrating experience at times, but that's partly because this is the first release of Unity and there's a few warts here and there that will surely get ironed out in future versions.
Incidentally, the announcement that Unity will be the default desktop experience for Ubuntu starting with Natty Narwhal comes mere days after my one week run.
So, just what's so good about Unity anyway?
More vertical space - Displays are going widescreen. That applies equally to netbooks which means vertical pixel real estate is much more valuable than their horizontal counterparts. Unity attempts to minimize on vertical pixel usage to maximise pixels available to apps.
Global Menu - This goes hand-in-hand with the above. The menu bar present in just about all apps takes up precious vertical space. Integrating it with the top panel saves just that bit more space. Space that the app can use to present more content to the user.
Touch agnostic design - I'm sure just about all Ubuntu users know about uTouch by now. With multitouch becoming more prevalent especially on devices with small screens, it's a unique opportunity to take advantage of touch capabilities for user interactions. Unity's UI design fits hand in glove with a multitouch paradigm.
Sidebar - Screens are going wide which gives us more horizontal pixels than vertical ones. Since that usually means some underutilised space particularly on websites, it just makes sense to put some of that real estate to better use. Unity's sidebar is much more compact and useful than the Windows Vista/7 sidebar, though that's just my opinion.
The first release of Unity isn't without flaws. There's rendering glitches every once in a while though that's likely a driver bug. Performance also isn't always consistent. Animation effects occasionally stutter, giving a less than stellar impression to the user. Firefox sometimes causes the entire desktop to "blank out" and refresh itself whenever it's closed. The shift from Mutter to Compiz should improve Unity on the performance front.
I'm not so sure Unity would make such a good desktop shell for regular desktops, but Natty is just starting development so anything's possible. Expected performance improvements with the move to Compiz 0.9 will definitely help a lot. Maybe I've just grown too used to GNOME Do, but I find Unity's interations rather clumsy. Providing a feature similar to Do for Unity would be interesting, I think.