How does it compare with NASA's WorldWind....

WorldWind runs circles around Keyhole viewer (Google) on my desktop....

I especially like the ability to switch instantly between photo and topological maps.... I can see a terrain item I'm interested in and then switch (if I'm looking at the US) to a USGS map with roads, navigational aids (if in coastal waters, etc.)

Extremely responsive with 3D graphics support... dog on old systems with limited graphics accellerators.