I played with the Keyhole client a long time ago: it allowed users to roam the earth viewing satellite images, zooming over my house, checking out where I worked, tilting the view so I could the images rendered on the contour of landscape (very impressive). It was all going so well... And then my 30 day free trial expired.
Though I enjoyed it, it didn't seem worth the $30/year subscription. I mean, it was just a toy right?
Fortunately for everyone, a few months ago Google (having acquired Keyhole) decided to improve and make the entry level client free to all. However altruistic Google is, I suspect they also believed that the free version of (the now-rebranded) Google Earth was the best advertisement for the commercial version.
So is it worth trying Google Earth?
Most definitely yes! And not just for the pleasure of flying over your neighborhood. Playing the virtual tourist is a ton of fun, esp. when all you do is type "Paris, France" in the search box and you're wisked off to Europe (in my case zooming out from North America, flying over the Atlantic, and zooming down on Paris). And once you're there, why not check out the Eiffel tower?
The software's real strength, as you'd expect from Google, is in the way it leverages the internet community. The Google Earth bulletin boards are filled with lots of destinations (double-click on them in the browser and they'll direct your Google Earth program to that spot, usually with notes describing what you're seeing) and overlays (imagine being able to see hurricane Katrina's before and after impact on New Orleans, or viewing storm forecast models).
My favorite is the National Geographic coverage of Africa and the data they've set-up for Google Earth. There are over 4,000 high resolution pictures that you can zoom in on, each represented by a little plane icon. Click on the icon once and you get a callout with a low-res picture and description of what there is to see. Double-click on the icon and you zoom into the picture. Check out the elephant family I found... And this isn't even the full resolution.