Are carbon hubs a good idea? I'm beginning to see manufactures making carbon hubs and am wondering if they have the potential to perform better than conventional metal hubs or if they are just a way to save a couple of grams of weight, possibly at the expense of performance. 

 Edit: 
These are the hubs I was originally asking about, manufactured by enve.  They have a full carbon shell and flange.  The flange is not drilled, the holes are molded as such.
Which hubs are you thinking about?

It would be very hard to make a carbon hub flange strong enough to withstand spoke tension, assuming you are building a wheel with conventional spokes.  Most carbon hubs I have seen use aluminum flanges, and the few which offer significant weight savings are ultralight equipment not for general use.

Carbon probably won't replace the axle, bearings, freehub, or quick release, either.  So you're talking about a few inches of tubing being carbon instead of aluminum, and you're trading the one-piece hub shell for something assembled.  The hub is subject to considerable stress from riding so it's not an area where it makes sense to compromise.