Well, I have used the online complaint portal at the link http://mis.bsnl.co.in/pgs/internet/pgwebregn.asp on various occasions. Most of the time the complaint has been resolved successfully.
But, ultimately the complaints are routed to your local customer service centre itself. Which would mean you need to have in the good books of the concerned officials. But, in my case the experience has been quite satisfactory.
However, please note that the portal expects you to have complained to the concerned service centre first. If the case has been unresolved for a longer enough time, you can file the complaint on the abovesaid portal.
You may also consider registering your complaints on the link http://www.tccms.gov.in/. This is the official link for Telecom Consumer Complaints Monitoring System.