Ceramic bearings - bearings with ceramic balls rather than steel balls - are a common choice by the top-end cyclists. So, how much do they really gain you?
If I can, I try to figure out what sort of maximum gains you might see when from a specific increase. In this case, we can do a decent job, as we know roughly how efficient bicycles are.
The best data that I've found suggests that modern drivetrains are about 95% efficient (though it's hard to know exactly what they're measuring). If we could get rid of all the lost, it would be like we increased power by 5%. So, a climb that took us 10 minutes would now take:
600 seconds * 0.95 = 570 seconds.
Which seems like an impressive difference.
Though, from a speed perspective on the flats, it's not as big of a deal. 5% more power takes us from 15 MPH to 15.3 MPH, or from 20 MPH to 20.4 MPH.
But, we're not going to get rid of all of that - there's still some loss in the chain.
Cyclingnews did an article a while back on SRAM's $190 bottom bracket that claims that it reduces frictional losses from 4% to 0.5%. Note that that also includes some other low-friction design changes and a low-friction grease. That's gets you up that 10 minutes hill about 21 seconds faster.
On the other hand, Zipp claims that you get 2 watts at 25MPH with their ceramic wheel bearings. The handy Speed and Power Calculator estimates that 25MPH requires 300 watts with hands on the drops, which means you're saving 2/300 or about 0.6% of your power. Not really a lot of savings there, though presumably you could save that much for each wheel, and gain a small amount of speed. They do note that the savings against the high-quality steel bearings they use on their other wheels is only about 1 watt.
We can also assume that both SRAM and Zipp are using the best ceramics they can find in their bearings, and there are very likely cheaper bearings that are going to have a lesser surface finish and therefore far fewer gains. I'd be especially leery of the improvement from other ceramic bottom brackets because my guess is that the seal design and lubricaton are significant factor, especially given the big difference in gains from using ceramic bearings between the bottom bracket and wheels. As for cost, I've seen 5 bearing sets for Mavic wheels for $300. Or, you can find a set of bearings for $35 on ebay.
In other words, buyer beware. Those ceramic bearings you saw on ebay may not be any faster than the ones you're currently using.
Speed Improvement: Medium (pretty good for a hardware change)
Coolness Factor: High (all the pros are doing it)
Cost effectiveness: Low
Verdict: The bottom bracket looks nice, but yowsa, those parts are expensive. But, probably a better use of funds that that titanium seat post bolt you were thinking of buying...