Ceramic knives are approximately as sharp as steel... but not for long.
In order for a knife to be a knife, the cutting edge has to be as small as possible, because physics. A steel edge will bend or warp slightly as it knocks against the cutting board, so that you're cutting with not-exactly-the-edge. Proper use of a honing steel (or strap of leather? I guess) will put it back in line so you can keep cutting. Ceramic knives are made of sterner stuff, so they don't need honing.
But both ceramic and steel knives will eventually have enough material stripped away from the edge that they become
dull. In this case, no amount of honing can resharpen a steel knife, you'll have to take a stone or a belt-sharpener to it to remove enough material from the sides that you have a proper edge again. And since they can't be sharpened, it is at this point that a ceramic knife is only fit to throw away -- you might as well use the spine of the knife instead of the edge.
The idea that ceramic knives never need sharpening is pure informercial-marketing trash.
The two people I knew who owned ceramic knives kept them in the knife drawer with all their other cutlery, so it didn't help that they also got chipped and dinged from being swung around with two dozen other pieces of steel of various shapes and sizes.
It's my general experience that forged steel (a rod is heated and pounded and folded into shape) makes a better knife than stamped steel (a plate has the knife-shape cut out of it, which is then ground to an edge), so a $70 knife does better than a $20 knife. But beyond that it doesn't make much difference, a $300 knife doesn't seem to be all that much better than a $70 knife.
Either way, you're better off with a $20 knife and a $10 honing steel than you are with a ceramic knife you got for free.
Edit: Fuck it.
Good Eats Moment - Happines is a Sharp KnifeThat's a condensed version of an episode on knife maintenance, in which Mr. Brown (who lives and records in Marietta, GA) consulted with the aforementioned Geoff Edges on his craft. It was a big "Hey! I know that guy!" moment for me.