Edit: Ed beat me to it!
It will divert air around the sides of the car instead of it tumbling around the aerodynamically dirty undercarriage. That's good. It should ideally stick down just far enough to shield the chassis and suspension components from oncoming wind, so it looks a little low to me. Perhaps cut an inch or two off the top edge and then install it. This is different for every car, so you'll have to eyeball it to see whether it's sticking down too much or not enough and modify accordingly.
As I understand it, downforce doesn't really make the car feel heavier as much as it just gives you more grip. It will increase your overall drag but make the car handle better. If you trim the thing right it will decrease your drag while maintaining basically the same handling. You're probably not going fast enough on the street for the downforce to make much of a difference anyway.
At the end of the day though you could probably get similar looks and the same aerodynamics out of black rubber lawn edging. Check out the Chevy Express van on here which did that and reported noticeable FE improvements.