The one we (flight sim enthusiasts) have been waiting for: Google Earth 4.2 incorporates a flight simulator.

On the plus side:

•  physics pretty good
• most realistic scenery available
• always blue skies
• amazing experience of the landscape
• easily controllable with the mouse & keyboard

On the minus side:

•  no sound
• have to pause on a regular basis to allow imagery to be updated
• the occasional crash (as in, program crash!)
• well visible that height info & textures don’t always align