Virtues of Zamzam Water
Question

What is the status of Zamzam water? What are its virtues? Why are Muslims so keen on Zamzam water?
Praise be to Allah.Status of Zamzam water
Imam Ibn al-Qayyim al-Jawziyah (may Allah 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 Jibril and is the water with which Allah quenched the thirst of Isma’il.
It was reported in Sahih Muslim that the Prophet (peace and blessings of Allah 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 (peace and blessings of Allah 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 (peace and blessings of Allah be upon him) said: “Verily, it is blessed, it is food that nourishes.” (Narrated by Imam Muslim, 2473)
Other scholars added, with their own isnads, “… and a healing for sickness.”
This was narrated by al-Bazzar (1171, 1172) and al-Tabarani in al-Saghir (295). 
In Sunan Ibn Majah (al-Manasik, 3062), it was reported from Jabir ibn ‘Abd-Allah that the Prophet (peace and blessings of Allah be upon him) said: “The water of Zamzam is for whatever it is drunk for.” The Salaf and ‘ulama acted upon this Hadith. When ‘Abd-Allah ibn al-Mubarak went for Hajj, he came to Zamzam and said, “O Allah, Ibn Abi’l-Mawali told us from Muhammad ibn al-Munkadir from Jabir (may Allah be pleased with him) that Your Prophet (peace and blessings of Allah be upon him) said, ‘The water of Zamzam is for whatever it is drunk for.’ I am drinking it to ward off thirst on the Day of Resurrection.” Ibn Abi’l-Mawali is thiqah (trustworthy) so the hadith is hasan (good).”
Virtues of Zamzam water
Ibn al-Qayyim (may Allah 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 Allah. 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 Tawaf 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 Tawaf numerous times.” (Zad al-Ma’ad, 4/319, 320)
Shaykh Ibn ‘Uthaymin (may Allah 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 hadith has been narrated concerning this: the difference between the believers and the hypocrites is drinking one’s fill of Zamzam water.” (Narrated by Ibn Majah in al-Manasik, 1017; al-Hakim, 1/472)
Al-Busairi said: 
“This is a sahih isnad; its men are mawthuqun [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 Allah 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 Allah to quench our thirst from the Cistern (al-Hawd) of His Prophet on the Day of the greatest thirst. May Allah bless our Prophet Muhammad.
And Allah knows best.