There is a free version and a pay version munroe. The pay version doesn't add anything usefull that the everyday person will need. Good old google!
It needs a broadband connection to run well as it downloads the images instead of storing them all on your pc (though it does a bit of that too). I suggest looking for your house when you get it too, that always fun.
Tips:
1. To find somewhere type the place you want to find in the "fly to" box in the top left (EG. type Area 51 in there and it will take you there)
2. Its not live! if you go outside and wave you wont see it. Some of the images are 5 years old some even more!
3. Check out the community for help and interesting places to visit and do (
http://bbs.keyhole.com) Keyhole is what it use to be called.
4. Have fun...and go exploring without leaving your chair!
5. Also try:
http://local.live.com/ - windows take on google earth (require no software)
http://maps.yahoo.com/beta/ - Yahoos version (also no software)
These have different image datebases to google and therefore you see different things in different ones.