There are many categories of rishis, said M.V. Anantapadmanabhachariar, in a discourse. There are kings who become rishis and are therefore called Rajarishis. There are Maharishis and Brahmarishis.
Viswamitra's ambition was to be known as a Brahmarishi. And towards that end, he did severe penance. The kind of penance he did would be hard even to imagine. Pleased with his penance, Brahma appeared before him and addressed him as “Rishi.” But Viswamitra was not satisfied. So he increased the severity of his penance, and now Brahma addressed him as “Rajarishi.” But still Viswamitra was not satisfied. So he did more penance. And then Brahma asked him what he wanted and why he was resorting to such penance. Viswamitra said he wanted to be acknowledged as a Brahmarishi. Brahma said that was possible only if Viswamitra exhibited the quality of patience. So he repeatedly asked Viswamitra whether he could be patient. This question put to him repeatedly annoyed Viswamitra so much that he lost his temper! So he had to go back and do more penance.
Finally, he did earn the title of Brahmarishi, but he was not happy. He wanted Sage Vasishtha to address him as Brahmarishi. And only when sage Vasishtha did so, was Viswamitra pleased. If Viswamitra had not persisted and continued with his penance, he would never have become a great sage, and if he had not become a great sage, we would not have got the Gayatri mantra. So the penance of sages is for the benefit of mankind.
Sages too had flaws. Even Viswamitra was tempted by the charms of a woman. But this only shows how careful we must be. If even rishis make mistakes, isn't there a greater possibility for sin in our case?
Sages have immense powers. Once there was a contest between two sages, about who was the superior of them. The test was to light up a wet log through their powers. One sage made the log give out smoke. But the other one was able to make the log catch fire and the log burnt to ashes. Thus the latter was declared the winner.
We are said to be born with three debts. One of them is a debt to rishis. This shows how much we owe them.