There has been a lot of marketing around ceramic bearings. Some have claimed massive and frankly unbelievable claims of speed increases.
I have been using and selling hybrid ceramic bearings for a number of years, and while I do believe they are better I don't believe these inflated marketing claims.
The commonly available "ceramic bearing" is in fact a "hybrid" bearing. What this means is that the races are made of steal and the balls of ceramic-typically silicon nitride.
The silicon nitride balls are smoother and harder than the steel balls used in conventional bearings. This hardness and higher tolerances/smoothness results in lowered energy absorption in the bearing.
Additionally the non metallic ceramic ball removes some of the failure modes of a steel ball which can make a hybrid bearing more robust than a conventional bearing. However because hybrid bearing typically use lower grade seals for even less friction contaminants can get into the bearing easier, and this can cause the steel races to still fail quickly. Quality seals and high grade steel designed for use with ceramic balls are vital for high quality hybrid bearings.
This is why hybrid bearings have developed a poor reputation for not lasting long. Lower quality bearings/brands have cut quality corners to keep prices down and this has resulted in poor durability. A poor quality hybrid bearing is just as likely to fail quickly as a poor quality designed steel bearing. However it hurts more when you hybrids fail quickly as they didn't cost you $5 each to buy!!
I am often asked by customers if they will feel an increase in performance if they upgrade their bearings. I'm an engineer and too honest to be a top salesman! I typically ask how old their wheels/bearings are and what brand wheels/bearing we are talking about. Some brands have better quality bearings than others to start with, and some hubs have better designs that improve bearing life and performance.
Simply put, if you have good new bearings, you may not *notice* much improvement. However if your bearings are starting to show their age and you are thinking of replace them, why not upgrade with the best?
I mostly noticed the increase in smoothness with my hub bearings when riding in a bunch. I was "soft pedalling" or free wheeling far more than those around me. This clearly suggests I didn't need to put in as much power as the person next to me to keep speed with the bunch.
So will you go XXkph faster and save YY seconds in a race? Sure *GOOD* hybrid bearings do roll a little smoother and can last longer than standard bearings.
If you've spend a lot on your wheels and ready to service your bearings, it makes sense to me to replace the bearings with something of quality to match.