I have disc brakes on my Giant Anthem mountain bike (Avid brakes). They have great stopping power but they require almost daily adjustments to keep them from rubbing slightly. They are the highest maintenence bike component I have ever owned. Even when they are adjusted perfect, there is less than 1 mm of clearance between the pad and the disc. I have even had them back to the LBS and they just say "disc brakes tend to rub a little, live with it." Maybe Mtn bikers can accept this but for a roadie, hearing this rubbing drives me crazy and is unacceptable.
They better learn how to make disc brakes more foolproof if they ever want my business on a road bike.