Yes, a metal build up impairs performance. The small pieces of metal are called swarf, and they have a tendency to mess with the abrasive qualities of your hone in the worst sorts of -- seemingly contradictory -- way. The swarf makes the hone less abrasive by clogging it, but can also lift off the hone and scratch and chip the knife.
There are a number of ways to clean a ceramic hone. You can use an abrasive like Comet or Ajax cleanser, and/or an abrasive pad -- in fact you should do this on a regular basis. Eventually though, that won't be enough and you'll want to use something like the Idahone "Superaser." http://www.japaneseknifesharpeningst...uctCode=IdaEra
Hope this helps,
PS. If you're looking for a good ceramc hone, you can't do much better than the Idahone 12" fine.