Everyone beats Karthus in lane.
Karthus is a strong pick in arranged team play. He is a hypercarry mage, much like Tristanna or Kog'Maw are hypercarry ADs.
He is going to be very weak in the laning phase, but toward the 25-30 minute mark, assuming you keep your farm up, you walk into a teamfight with 'e' up, drop some 'q's until you melt, press 'r' and get a triple.
Once you hit 6, make sure you have blue and watch other lanes for your R. Once you have blue, you are unstoppable farming machine. Push mid back and gank other lanes. Farm every piece of jungle on your trips and just have at it. A bad game for Karthus should be equal CS with mid and not having the oppurtunity to use R.