Three Myths about disk brakes

Three Myths about disk brakes

3.) Disk brakes are better than rim brakes because rim brakes only work their best on perfectly true wheels.
Not false, but disk brakes require perfectly true rotors.  If you're not careful where you lean your disk-braked bike, or you crash hard, you'll need to true a bent rotor.  Even slightly out-of-true rotors will make a big racket and reduce your braking effectiveness.

A rotor-truing tool.

2.) Disk brakes work better in wet weather than rim brakes.
Only in seriously muddy situations.  The disk rotor is kept farther from the grime, but it is still possible to splash mud and crud into the system and contaminate the braking surface.  This will cause squealing that can only be eliminated by scrubbing your rotors and brake pads with alcohol.  In contrast, a well set-up rim brake with a wet-weather capable brake pad (Kool-Stop salmons, or my favorite SwissStop green compound) will work just as well wet or dry, with perhaps a little more noise in the wet.

1.) Disk brakes are more powerful than rim brakes.
Depending on application and use.  Disk brakes are prone to losing strength when they heat up, moreso than rim brakes.  The reason is simple.  For equal bikes with equal stopping capabilities, the amount of energy dissipated by the brakes is the same.  The rim brake bicycle deposits this energy into the wheel rim, and the disk brake deposits this as heat into the rotor.  The rim is a bigger chunk of metal than the rotor, so it's temperature rises less for the same amount of energy.  Heat lessens the friction between the pad and the metal producing an effect called "brake fade".  Faded brakes take more pressure on the lever to get the same stopping power.

During long, fade-inducing mountain descents on my late disk-equipped Kona, I would sometimes stop and hose my rotors with water from my hydration pack.  It's pretty impressive to see the water boil instantaneously, but less impressive to accidentaly press your calf against the hot rotor while stopped.

Proper braking technique can help reduce disk brake fade.  Using your brakes in short, powerful bursts rather than long, slow drags will reduce heat buildup.  This is natural on mountain trails, but for road riders who are prone to dragging brakes on long hills, fade can be an issue.  In fact, roadgoing tandems designed for long descents replace the rear disc brake with specially designed drum brake in conjunction with a rim brake.  The rim brake is used for routine stops on flat ground, and the drum is used downhill braking.

A tandem drum brake has a large metal heatsink to dissapate heat for fade-free braking. 

The bottom line: it doesn't matter so much which system you choose, as long as you buy quality components, have them set up properly by an experienced mechanic, and use proper braking technique on big hills.