The program lets you do smooth sailing flybyes of the entire Earth. You can easily fly to any spot on the globe, by entering any associated data, like street addresses, place names or lat/long coordinates.Nathan posted some amazing screenshots of the 3D buildings, fly through directions, and tilted satellite image views.
It will be interesting to see how much of this gets integrated into Google Maps.
Update: Bill Kilday has the post about Google Earth on the official Google weblog.
Update: I have installed it and... wow. Wow, wow, wow. This is like Google Maps on steroids. The flybys, rotations, and tilts combined create a jaw-dropping experience. Go download it now. It really is remarkable.
Update: Rob Pegoraro at the Washington Post has a detailed review of Google Earth. [Found on Findory]