Yes, ceramic knives are the "new thing," but that doesn't make them superior. The problem with ceramic knives is that you can never sharpen them, and, as mentioned in the comments, they may chip. Don't get a ceramic chef's knife or paring knife; the answer to your question is zero.
If you really want a ceramic knife, then buy a ceramic bread knife, although you won't be getting any extra performance for the money.
Ceramic blade mandolines, however, are great. The ceramic blade will be sharper and hold the edge longer than a steel mandoline, and since you don't sharpen a mandoline anyway, it doesn't matter that ceramic doesn't sharpen.
Note, a separate, and good, question would be "what knives do I need?" The answer is it depends, but I concur that you should not buy a "set" of knives.
Take them to a professional. Nothing you can affordably buy in your home will work well.
Personally, I don't buy quality serrated knives. I buy cheap and replace when dull. Only my normal blades are quality, and these I have sharpened yearly.
Best Answer
Ceramic blades can only be sharpened against something that is harder than it is. I would recommend sending it back to the manufacturer for sharpening if needed. Typically it is done using a very hard abrasive wheel because of the risk of the blade breaking, you do not want to do this without proper safety equipment.