Well, for what it's worth, I preferred Avid's Single Digit series over their overly-complicated Arch series. Under ideal conditions the Single Digits stopped me fine. Under wet, or extended / dusty / steep conditions, I never had enough power.
But discs (BB7s) surprised me not so much due to their superior, consistant power, but because of their excellent modulation. It was the first time I never wanted anything more from my brakes.
Every rider is going to be different based on weight and terrain and riding style / technique, but if you're pleased with your current brakes, there is really no reason to look to make the switch... unless you're curious.