Narrated Ibn Abbas (radiallaahu `anhu):
Allah's Apostle (sallallaahu `alayhi wasallam) came to the drinking place and asked for water. Al-Abbas (radiallaahu `anhu) said, "O Fadl (radiallaahu `anhu)! Go to your mother and bring water from her for Allah's Apostle (sallallaahu `alayhi wasallam)." Allah's Apostle (sallallaahu `alayhi wasallam) said, "Give me water to drink." Al-Abbas (radiallaahu `anhu) said, "O Allahs Apostle (sallallaahu `alayhi wasallam)! The people put their hands in it." Allah's Apostle (sallallaahu `alayhi wasallam) again said, 'Give me water to drink. So, he drank from that water and then went to the Zam-zam (well) and there the people were offering water to the others and working at it (drawing water from the well). The Prophet (sallallaahu `alayhi wasallam) then said to them, "Carry on! You are doing a good deed." Then he said, "Were I not afraid that other people would compete with you (in drawing water from Zam-zam), I would certainly take the rope and put it over this (i.e. his shoulder) (to draw water)." On saying that the Prophet (sallallaahu `alayhi wasallam) pointed to his shoulder.
Imaam Ibn Qayyim al-Jawziyah (may Allaah have mercy on him) said:
Zamzam water is the best and noblest of all waters, the highest in status, the dearest to people, the most precious and valuable to them. It was dug by Jibreel and is the water with which Allaah quenched the thirst of Ismaa’eel.
It was reported in Saheeh Muslim that the Prophet SAWS (peace and blessings of Allaah be upon him) said to Abu Dharr, who had stayed near the Ka’bah and its coverings for forty days and nights with no food or drink other than (Zamzam): “How long have you been here?” Abu Dharr said: “I have been here for thirty days and nights.” The Prophet SAWS (peace and blessings of Allaah be upon him) said, “Who has been feeding you?” He said, “I have had nothing but Zamzam water, and I have gotten so fat that I have folds of fat on my stomach. I do not feel any of the tiredness or weakness of hunger and I have not become thin.” The Prophet SAWS (peace and blessings of Allaah be upon him) said: “Verily, it is blessed, it is food that nourishes.” (Narrated by Imaam Muslim, 2473).
Ibn al-Qayyim (may Allaah have mercy on him) said:
Myself and others tried seeking healing with Zamzam water and saw wondrous things. I sought healing with it from a number of illnesses, and I was healed by the permission of Allaah. I saw someone who nourished himself with it for a number of days, half a month or more, and he did not feel hunger; he performed Tawaaf along with the other people just as they did. And he told me that he consumed nothing but Zamzam water for forty days and he had the strength to have intercourse with his wife, to fast and to perform Tawaaf numerous times.Zaad al-Ma’aad, 4/319, 320.
Shaykh Ibn ‘Uthaymeen (may Allaah preserve him) said:
So you should have the intention of what you want to gain by drinking this water. He should drink his fill, i.e., fill his stomach with it until it is filled to the ribs, because this water is good. A hadeeth has been narrated concerning this: the difference between the believers and the hypocrites is drinking one’s fill of Zamzam water. (Narrated by Ibn Maajah in al-Manaasik, 1017; al-Haakim, 1/472).Al-Boosairi said: this is a saheeh isnaad; its men are mawthooqoon [trustworthy].This is because Zamzam water is not sweet; it is somewhat salty, and the believer only drinks this somewhat salty water out of faith, believing that there is barakah (blessing) in it. So when he drinks his fill of it, this is a sign of faith.(Sharh al-Mumti’, 7/377, 378, 379).
Perhaps Allaah did not make it sweet so that people would not forget that the meaning of drinking it is an act of worship. Whatever the case, its taste is fine and there is nothing wrong with it. We ask Allaah to quench our thirst from the Cistern (al-Hawd) of His Prophet on the Day of the greatest thirst. May Allaah bless our Prophet Muhammad.
Taken from IslamQA.
Taken from: http://ilookilisten.wordpress.com/2013/03/10/virtues-of-zamzam-hadith-no-1400/