I have both in my stable and swap between the two all the time. In dry weather I don't notice much difference. The rim brakes never have as much power, esp calipers but I can stop without too much effort. V-brakes are close to disc in terms of braking power in dry conditions. In the wet, there's no comparision. With rim brakes I have to start braking very early to clear the rim's braking surface and the friction is reduced greatly so braking power is very poor (caliper, canti's or v's). I went off-road on a muddy trail the other week with a rim braked bike, I had to ride much slower due to limited brakes. I had forgotten how bad they could be. With disc brakes in the wet/mud I have much more braking power. There's so much difference that I am switching my 3 season commuter to front disc this winter. I really like disc brakes.

"braking power" = how hard I have to pull the brake lever to slow down