Google Maps serves its purpose, which is entirely different from Google Earth. Where you might go to Google Maps for directions or even a satellite view of a city or neighborhood, Google Earth takes you on a flight, an exploration of places you may never be able to see any other way.
Take a look at the globe that is gathering dust on your desk or bookshelf. Now imagine being able to look at that same globe with a microscope, zooming in on countries, states, cities, neighborhoods... all the way down to your house. Next, imagine flying from your house on this dust-catching globe to, say, the Great Wall of China, in a matter of a few seconds. While at the Great Wall, why not stay a while and watch a documentary about it? All of this and more can be done in Google Earth.
Besides the obvious educational uses, it's just plain fun. I started my trip at my childhood home, flew up the street to the hill where my friends and I hiked, rocketed over to the East Coast to take a look at the Statue of Liberty, and ended my journey with a tour of my birthplace in Peoria, Illinois. A separate trip took me around the world: London, Australia, China, Japan, Paris... even Walt Disney World.
Aside from flying anywhere on the planet, Google Earth offers more technical features such as altitude and GPS coordinates, among others. Pair that with the less-obvious multimedia features--including user-submitted photos of landmarks and downloadable videos--and you have a full-featured program that can provide you with hours of exploratory and educational fun.
The only apparent drawback to Google Earth is the occasional "limited visibility". While exploring Peoria, there was a small area that appeared only as large pixels, rather than the usual impressively high detail. Though this temporarily reminds you that you are not, in fact, flying, but are instead seated all too comfortably in front of your computer, you will be distracted soon enough by your next destination.
After hours of touring the "world", you'll want to join the Google Earth community. And don't forget to send a postcard home.