The rear brake is O.K. for situations where traction is poor, or for when your front tire blows, but for stopping on dry pavement, the front brake alone provides the maximum stopping power, both in theory and in practice.
If you take the time to learn to use the front brake correctly, you will be a safer cyclist.

Many cyclists shy away from using the front brake, due to fear of flying over the handlebars. This does happen, but mainly to people who have not learned to modulate the front brake.

moar-> http://www.sheldonbrown.com/brakturn.html