My wife has disc brakes on her old road bike (Redline Conquest D) and her new road bike (Lemond Poprad) and absolutely loves them. Insists upon them actually.
First tried them in hydraulic form on her mountain bike (Giant XT-C), and quickly went shopping for a road bike with discs. Both of her road bikes have Avid mechanical disc brakes, and both work quite well.
Yes, they are a bit heavier, so I don't think you'll see them in the pro peloton, but for normal road use they work great.
Rick / OCRR