If I can add just a bit of controversy in this as well - drum brakes actually stop quite well. There is far more friction material in a drum brake than there is in a disk brake - look at the typically small pads (albeit one on either side) only gripping a small percentage of the brake disk - while a drum brake has two massive shoes that cover approximately 80% of the available area, which tends to be larger than that of a disc.
The problem with drum brakes is that they don't shed heat so in REPEATED braking, they lose effectiveness very VERY quickly. Notice that Wikipedia doesn't mention inferior stopping power as a disadvantage - it's brake fade due to heat that killed them.
cheers!
