"It is the nature of the Hare Krsna maha-mantra (the great mantra of deliverance) that whoever chants it immediately develops loving ecstasy for krsna." (Chaitanya-caritamrta, Adi-lila 7.83)
Unadulterated love for Krsna is eternally present in dormant state in the hearts of all living entities. When hearing and chanting purify the heart, the living entity naturally awakens. The processes of hearing and chanting-sravanam kirtan purify the heart, and one's original Krsna consciousness is immediately awakened. When one chants the holy name of the Supreme Personality of Godhead, the heart is cleansed of all material contamination. In Brihanaradiya Purana it is mentioned that in the age of Kali Yuga, people in general are slow in self-realization and perpetually disturbed with material anxieties, and therefore the recommended means is to simply chant the holy names of the Lord.
The transcendental vibration established by the chanting of Hare Krishna Hare Krishna Krishna Krishna Hare Hare Hare Rama Hare Rama Rama Rama Hare Hare is a sublime process that revives our original Krsna consciousness. Chanting awakens within us the dormant love that everyone has for the Supreme Personality of Godhead, bringing great happiness and ecstasy. Chanting is simple but should be performed properly for best results. One must chant loud enough in such a way that atleast someone sitting nearby can distinctly hear each syllable. While chanting, one must concentrate the mind on hearing the mahamantra. This concentration is mantra meditation and is so powerful that it can cleanse the lusty desires accumulated in the heart for millions of lifetime. It is however, difficult to stop the mind from wandering, but as with anything else, practice makes perfect.
The best time to chant is early in the morning (during the brahma-muhurta, the auspicious period before sunrise). One can chant in any circumstance, but it is best to best to finish sixteen rounds with full concentration early in the morning, before starting our routine daily activities. In this day and age, performing anything with dedication and for an extended period of time possibly life long, especially if it has nothing to do with the material world, however one must be merciful to oneself and dutifully complete one's rounds without letting any sorts of material distractions create hindrance.
There are offenses involved in chanting too. If one chants the Hare Krishna maha-mantra but his life is still full of sinful activities, the loving servitude position to the Lord remains elusive to him. But if in spite of being an offender one chants the holy names of Gaura-Nityananda, he is very quickly freed from the reactions to his offenses. Therefore one must first take shelter of Sri Caitanya Mahaprabhu by chanting Sri-Krishna-Caitanya Prabhu Nityananda Sri Advaita Gadadhara Srivasadi Gaura Bhakta-Vrnda. (Caitanya-caritamrta, Adi 8.31 purport) before chanting each round of the Hare Krishna maha-mantra.
Chanting is also purifying for anyone who hears. That is why public chanting should take place wherever possible, so that even those who are unfortunately not interested in this great means of deliverance can receive inadvertent benefit simply by hearing accidentally. Such is the power of this incredible means of deliverance for the fallen conditioned souls of Kali Yuga.