We use one in our Mustang.
At first we were buying the shaft for the weight savings and for safety. After getting the CF shaft and comparing weights to the aluminum shaft it was noticed that there was no weight savings. As for safety, the CF shaft is far safer than a steel or aluminum shaft. Many people including us have broke a CF shaft, instead of ripping through the steel floor they splinter in small pieces.
Now there are a couple of down falls to CF shafts, first is the cost. Some people have had bad luck getting the CF shafts to hold up in any horsepower combinations. In fact we twisted the aluminum yoke inside the CF shaft on the first outing with it. The next issue which is the biggest problem. The CF shaft is extremely fragile to any impact, not so sure about rocks but any banging into a solid object will cause a weak point. Because of this impact next time you use your driveshaft, it will splinter into pieces. As for safety of CF splinters getting underneath the tires at the big end, this hasn't been an issue with any of the cars. Typcially, they will let go at the first 100ft of the track.
As for performance gain there is none.
Jeff Jr.