I am using airtel for around 6 month in mumbai. Service is good but not best one which they claim at least in mumbai. I found out many places where vodafone was giving better service then airtel.
It’s not that they are bad or I won’t recommend them, but they are not the best, but charges are at par with best network.
Customer service is excellent and get immediate reply. They charge for talking to executive but charges are nominal and worth it. Data connection works well. I have used 3G and 2G. 4G is again not avaliable everywhere in Mumbai, but 3G works well. No billing issue yet.
What I understand is vodafone has some technical advantage in terms of bandwith and cell location as it is oldest network in mumbai. But in that case Airtel should provide some incentive to cuatomer for not providing best network.