I folded last week and bought myself a SpaceNavigator 3D mouse from Amazon after reading great reviews of it. They have been out a little while but only recently has the company released a Mac driver, and Google Earth added support for it in 4.1.
Flying around Earth is likely to be my main use of this great little peripheral.
Since I first opened up Google Earth I didn't find it completely intuitive to navigate, but I got used to it. Mice and trackpads simply weren't designed for this purpose.
The SpaceNavigator is designed exactly for it though, giving you so much more power in navigating a 3D globe. I have already spent hours just flying around Stuttgart's valleys and even trying to land at its airport - I wasn't too successful - as well as flying around the Isle of Man and exploring parts of Japan and Manhattan where there are 3D buildings available to create an imersive world through which to fly.
I'm not sure I have much of a practical use for the SpaceNavigator at the moment, but it's great for just allowing yourself to get lost exploring the world from the comfort of your laptop. And with Amazon offering to pay 20EUR of it just for me to use their credit card, it was difficult to stop myself getting this great little navigator.