well, a couple things is that it's expensive. and it's not really going to help much, it's more of a visual thing. yea, they will grip better but majority of our braking comes from the front of the car anyways. also, you will need to have at LEAST 17" rims to clear the calipers. if you're doing it for looks, i say go for it. if you're looking for better performance, i would suggest swapping out the fronts instead.
If I could afford to upgrade my brakes i would, but i would rather spend that money making my car a show quality car, or at least a lil more tasteful than that god-awful light grey interior that my car has, and its filthy, i wish i could do something, but its not happening without some money.