I have one, I got it a few years ago when they changed the law and required passport or passport card(and now they offer enhanced license) to cross the border into Canada. That was before I decided I wanted traveled out of the US, and then I had to get an actual passport too, and then I kicked myself for wasting money on a passport card.
It's only allowed for land and sea travel to Canada, Bermuda, Mexico, and the Caribbean, so I don't think it's much use unless you go on a lot of cruises or drive to Canada or Mexico a lot. That being said, I do find it handy because I go to Canada to visit friends from time to time, and its easier to carry around a plastic card rather than carry around my passport over there.
It sounds like you are asking about getting a passport card instead of a passport? You cannot fly using a passport card.