Worthless depending on your perspective. They stop the car and the rears do so little you most likely won't notice the difference in stopping power. The organics just can't take the heat. Once they get hot the pads dissapear quickly. They also are super easy on the rotors. The semi's easily last twice as long but they wear the rotor a little more. I have yet to kill a rear rotor due to heat or time on track so I go with what lasts. Now if your calipers are sticky you can toss all of this out the window as no rear pad is going to last a full day (4 lapping sessions) dragging around the track. I always inspect, clean, and normally turn in the pistons to make sure they won't drag before every track day.
Don't forget some sort of front brake cooling ducts..
