http://www.autoblog.com/2006/07/28/carbon-fiber-wheels-coming-to-a-tuner-near-you/

The low weight but high stiffness of carbon fiber rims produces a very light weight rim to the wheel ratio, significantly reducing the moment of inertia and drastically reducing the amount of power required to turn the wheel. Consider that a standard Porsche 18" wheel weighs about 30.86lbs, the DYMAG CARBON FIBER/MAGNESIUM equivalent weighs around 13.23lbs for a total weight savings of approximately 70.55lbs per package!

if they looked good and i could afford them, i'd buy a set

Not sure how strong they would be. We have a CF expert on this forum. I'll wait for him to chime in. :)

Right now, i think they're only good on motorcycles.
IMO, they won't catch on with street cars for a while. If race cars start using them, they'll solve any potential issues first. The Mosler is supposed to be using them soon, but i think it's one of those new things that will soon fade away.
Sounds great and all, but there's a reason magnesium wheels are still being used. If F1 cars or planes start using them, then consider them.
When they are good enough for normal street cars, it will be great. Very light weight increases acceleration and braking, and carbon won't transfer much brake heat to the tires.