There are a few differences between carbon fiber and aluminum driveshafts. 1) Carbon Fiber is lighter than aluminum and up to half the weight of steel. The lighter rotational mass will put less stress on the engine and more power to the rear wheels. 2) Carbon Fiber has excellent vibration damping characteristics. Not only does this transfer into a smoother ride, but also more power and potentially longer life for your engine. Consider the effects of a good crank damper. A carbon fiber driveshaft could effectively absorb vibrations like a crank damper, but at the rear of the transmission/engine. 3) Carbon fiber driveshafts are typically wound in a way that makes them stronger than steel or aluminum when twisted normally while driving. The carbon fiber is cured in a very hard epoxy that will usually resist rocks and chips. Whether or not the addition cost is worth it is really up to you. If weight and strength are a factor, then carbon fiber might be the way to go.

One other HUGE thing that makes CF a driveshaft good besides all of the above is that if they do fail, they fail in a way that will destroy the shaft before it destroys the car and you for that matter. They essentially just shred themselves up before they rip up the floor boards and if you have ever seen the effects of a steel drive shaft come apart, it is a mess. You do get the $$ shaft a bit, they are expensive if you go CF. A good alternative is aluminum which is almost as light and no where close as expensive, but if you have some extra $$ CF is the way to go. Below are a few links to some of the makers, as well as a general page on driveshafts that show much of the differences between steel, aluminum, and carbon fiber (critical speed, etc).