pure ceramic bearings are very hard materials, so they are difficult to produce. if i recall correctly, there was an article in a motorcycle magazine, one from summer i believe, and they installed new ceramic wheel bearings on a busa, and they claimed from this alone they could tell a dramatic difference. they were about $100+ a set though, if memory servers me correctly.
i would think the lifespan would be much greater than steel bearings, due to the hardness and resistance to wear compared to regular steel bearings.
a major disadvantage to ceramic is the brittleness of the material. if the front end were to hit a pothole very hard, then the impact could, in theory, cause a ball or race to crack and this could lead to catastrophic failure. now i'm sure if you're going to pay upwards of $50 a bearing, there's been extensive testing done to almost insure this kind of failure doesn't happen. just my $.02