It has the drawback of not integrating very well with most of the current games because the game's UI itself needs to be drawn in 3D perspective. For example if you don't do that then you'll see two crosshairs when you focus your vision in the distance. And if you focus on the crosshair, you'll see everything else in double vision.
But that's minor. You should experience Google Streetview with it. You can google Eiffel Tower, look up, and almost feel like you're there.