[Archangel]

Its a subtle difference, but its definately there. I put both of mine on at the same time, so i can't speak specifically about the rear bar though.

the best description i've heard is:
It helps the car settle into turns quicker.

You should notice it when you first start turning, especially if you turn quickly. Without the bars, there is a little transition phase before the car feels completely stable, the bars eliminate that.

And no, like they said, don't jack up the car to put it on.