Actually, a hadith (saying of Muhammad, peace be upon him) is rejected if it directly contradicts the Qur'an. Rather, the Qur'an is understood through the words and teachings of Muhammad, as recorded in the hadith. Generally, the hadith bring the vague and inclusive statements of the Qur'an into the context of specific events or situations. The hadith can indeed be challenged, and there is rigorous debate around the authenticity of various hadith, but once authenticated, we accept the words of Muhammad (pbuh) as revelation on par with the Qur'an. Nobody, however, can override the Qur'an, and we Muslims don't actually have a formal system of priesthood, but rather scholarship. The only difference between one Muslim and another is in how educated they are (and how pious they are).