They're terrible cold and as such aren't suitable for street use. Steel brakes fade at the temperature that carbon brakes work at. This is a big difference in unsprung weight and is also useful for lowering the weight of an F1 car to the limit. Carbon discs and pads are much longer lasting than steel ones. In F1, they'll go the entire weekend even for a big buck team.
Smaller teams find that the rotors last three weekends.
In the 24 hours of LeMans in '91, the Mazda only changed its pads once, as opposed to 17 times for the Porsches.