check out the BST threads on speedzilla. the lightweight wheels argument has been raging on that site for a long long time.
and there's good arguments on both sides (but mostly just a lot of name calling). plenty of people who have used them on the street and the track long term with no problems whatsoever. but there have also been a few cases of the CF wheels failing with pretty severe reprecussions.
as a mechanical engineer, carbon fiber scares me a little when it is used as a structural part. I'm sure the people making the CF wheels and other parts are both very smart, and very good at what they do, but CF just seems like an unknown quantity. There's so many variables that go into the manufacturing process. There also seems to be a lack of evidence to how well these products are tested. Compare this to aluminum, steel, titanium, and even magnesium to an extent. there are mil standards that go back 100 years for these materials. published stress/stain curves for every alloy imaginable. tried and true methods for casting, machining, forging, etc. up against this, CF is relatively new on the scene. it's wide use in aerospace will probably change that though, as it's already spreading to automotive uses pretty quickly.
so what's my point? I dunno. carbon fiber wheels are really pretty. I can't afford them though, so it's really a moot point. also, it's friday afternoon, and I needed to kill a few minutes before I left work. [cheeky]