bsnl has cheaper plans than airtel and less nuisance. if you are in one of the circles where they are offering 100/175GB plans.
Unfortunately, it's not the case with Chennai circle. It seems to be different from Tamilnadu circle where ULD 2150 offers 175GB @4mbps. In Chennai, it just offers 100GB @1mbps. This is why I'm still sticking with Airtel.
As far as Chennai is concerned, Airtel has the best plans for heavy users. Reliance seems to be a good option in the last few days but it seems to be coming with it's own set of problems.
I posted this message before reading your last message. But like I said Chennai circle is not in TN circle. A few months back, I went to BSNL office and noone seems to have heard about these new plans.