All the pros on big budget teams use ceramic bearings because they are noticeably smoother - all you need to do is spin the cranks without a chain before and after to see what we mean. The Chris King Ceramic Bottom Bracket for SRAM GXP is the same as their stainless steel bottom bracket with beautifully machined cups but has ceramic balls in place of stainless steel.
The bottom bracket is constructed from precision machined, anodized alloy cups fitted with 24 ceramic bearings. These bearings can have grease purged and injected with Chris King's BB Grease Injection Tool just like the stainless steel bearings. So essentially, they are sealed bearing that are serviceable. Quite possibly the best bottom bracket available.
Threads on the cups are absolutely perfect, so they will install to your frame without issue. Center sleeve is attached to the cups with O-ring seals and succeeds in keeping dirt and condensation out of the bottom bracket assembly. The total weight is 101 grams including 85 grams for the cups and bearings, 8 grams for the spacers and 8 grams for the sleeve.