When you buy a controller specifically to make navigation easier.  ExtremeTech reviews 3Dconnexion’s SpaceNavigator, a device perfect for moving around in 3D spaces like Google Earth.  It won’t do you much good in X3:The Reunion, but for other, less action packed galaxies, it makes a very interesting controller.
“As we get closer to operating systems that hint at full 3D immersion during daily use—think Windows Vista—the idea of a 3D controller starts to become interesting. These 3D controllers, usually referred to as “6DOF” (six-degrees-of-freedom) controllers fully understand a 3D space. This may sound odd when you’re navigating on a PC’s flat screen, but anyone who’s played a game that really uses a 3D environment, Relic’s Homeworld real-time-strategy game, for example, often wishes for easier navigation through its virtual 3D space.”

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