I learned to drive off-road well before I ever learned on-road, and have always had something similar. I don't use it to gauge my decent.
This one I bought on Amazon for like $15 bucks. There is another model that has a compass (navigation type, not the jeep type) in the center, but from what I've heard from other Jeepers is that it's not accurate. This basically works the same way as a carpenter's level.
It won't replace good old common sense, but then again the same can be said for any instrument, gauge or readout system. (EVIC is a great example) I love the way it looks, and that little void right there to the left of the steering wheel seemed an ideal place to stick on.