whats the importance of Shodasi mantra in Sakteya sect? what benefits if a person starts chanting shodasi ona daily basis, and what -ve effects , if he misses it ?
This mantra is a form of Panchadashi mantra of Lalita Tripurasundari. In addition to the 15 letters, a single beeja is added to the mantra. This mantra would differ from person to person as the last beeja is given by Guru during higher deekshas. in the Sri Vidya tradition, this mantra holds a prime position hence it is a key mantra in Shakta tradition. However, it is not generally recommended to chant it without initiation as it may not work properly. Traditional people suggest importance of Guru deeksha for chanting and success of this mantra.
Yes it is able to give all the materialistic as well as spiritual benefits. There is no such thing that it will backfire if it is missed for a single day. But it is better to receive it from a good guru and then practice it regularly. In Sri vidya tradition, it is not given straight away but in 3rd or 4th stage of the worship. That is done as a krama deeksha. Step by step various mantra initiations are given to the sishya and Sodashi mantra is one of the mantras of higher deeksha (stage of worship).