Though, there are numerous scriptures have glorified the name of Lord Shiva in detail but since you are asking for specific gain of chanting his name, so I am answering only that. By chanting the five-syllabled (Namah Shivaya) mantra, even the fallen people (those who do not deserve holy rites) can be liberated. It has been said by Lord Shiva himself in Shiva Mahapurana, Section 7.2, Vayaviya Samhita, Chapter 13, Translated by J. L. Shastri.
Yes, surely, if the fallen man, under delusion, were to worship me with other mantras, excluding the five-syllabled one, he is sure to fall into hell.
Persons who live on water or air and those whose bodies are emaciated due to holy rites do not attain my region through those holy rites.
But he who worships me even once with devotion repeating the five-syllabled mantra, attains my region through the weightiness of this Mantra alone.
Hence, penances, sacrifices, observances and holy rites are not equal to even a croreth part of the worship with the five-syllabled mantra.
Indeed he who worships me with the five-syllabled mantra becomes liberated if he is in bondage.
He who worships me even once with the five-syllabled mantra with or without the Rudra mantra is liberated even if he is a fallen or a foolish man.
O goddess, he who worships me with the six-syllabled or with the five-syllabled mantra, with the Vedic mantra as its ancillary, with devotion to me, is liberated.
Some other glorification of five-syllabled mantra is present in Shiva Mahapurana, Section 1, Vidyeshvara Samhita, Chapter 17. In this chapter it is mentioned that by chanting this mantra one can be freed from indebtedness.
36-37. The months of Māgha and Bhādrapada are the most auspicious of all occasions. During the days of Japa be shall take only a single meal during the day in limited quantities. He shall abstain from useless talk and curb all his sense organs. He shall uninterruptedly render service to his parents and the king, or any master whom he serves. By performing the Japa a thousand times, he shall be free from indebtedness, otherwise not.
According to Shiva Mahapurana, Section 7.2, Vayaviya Samhita, Chapter 12, all sins will be wiped off by chanting five-syllabled (Namah Shivaya) or six-syllabled (Om Namah Shivaya) mantra.
- A person steady in the Japa of the five-syllabled mantra is released from the cage of sins whether he be a Śūdra, base-born, fool or a learned man.
Also, it is said by Lord Krishna in Mahabharata, Book 13: Anushasana Parva, SECTION XVII at the end of thousand names of Lord Shiva that:
"Krishna continued, 'That person, O Partha, who recites this hymn with a pure heart observing the vow of Brahmacharyya, and with his senses under control, regularly for one whole year, succeeds in obtaining the fruits of a horse-sacrifice. Danavas and Yakshas and Rakshasas and Pisachas and Yatudhanas and Guhyakas and snakes can do no injury to him.'"
