I first read about Keyhole a while back, and was thinking about downloading the demo to play around with. Then Google Maps introduced satellite imagery, and so I was satisfied with just that. But that doesn't give you the full control that the actual Keyhole software does; Keyhole lets you fly around the world, zooming in and out of cities, and get a feel of the terrain. I watched the video about MSN Virtual Earth, and that won't let us do that either, though it is extremely cool that you can smoothly zoom in and out of the top-level map.
But now I've discovered my new favourite toy, NASA World Wind. This is extremely cool, and is completely free (gotta love open-source.) While the Keyhole screenshots seem to show better resolution, this thing is still exceedingly cool. Definitely worth checking out.