I am a Airtel Delhi user for past 4 years and only problem I see is that their signal is quite weak inside the buildings / basements.
After their upgrade to new billing software, their billing cycle has not yet stabilised (its been 4-5 months now).. they still goof-up with the bills...
But I gather from my friends.. they are still better than Hutch.. the other big provider in Delhi.. so Airtel continues to be my choice...
They provide free connections to old (not Age!!) subscribers and have loads of services for heavy users...
One strong point surely Airtel has is that they have the best city coverage... I travel to differnt parts of he country and I am almost 90% connected.. this is really commended..