Maharishi's credibility is highly questionable.

He claims to have been Guru Dev's favourite student, the sole heir to his divine knowledge, but this is highly unlikely, according to Dev's other students, as Maharishi is/was not a Swami. This doesn't mean that he wasn't taught by Dev, but he would not reveal the knowledge unique to Shankaracharyas to Maharishi, because Maharishi could never hold the seat. And it was Swami Satchinand Swaroop who ended up getting this position. Swaroop insists that Maharishi was nothing more than a servant who received no instruction from Guru Dev and that he thinks he may have even been involved in the Guru's death.1

Vedic law forbids the Maharishi from giving out mantras and this transgression is further compounded by charging money for the service. The bija mantras he gave out, which are supposed to be meaningless, were not meaningless at all; as one website about TM says, "When is a Meaningless Sound not Meaningless? When it's the Tantric Name of a Hindu God."

The more I learn about Indian culture, Hindu, Hindi, Sanskrit and TM, the more I think Maharishi was either having a good laugh over the trusting westerners lining up to pay him for this and/or was simply interested in financial gain.

However, this doesn't mean that nothing positive came out of TM! There's even contradiction between my own parents who both studied with Maharishi and were TM teachers.. my mother heartily considers TM a cult, and my father thinks it was an immensely positive experience (which, he reports, greatly helped him kick the junk habit he had as a young adult.) There have also been studies which I consider trustworthy that indicate that doing TM raises one's IQ significantly; however, i consider these things to be the benefits of meditation, which perhaps was hindered by Maharishi's style of teaching rather than helped by it.

1 Hey, I'm not accusing anyone, that's just what he thinks.