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.