I have using Airtel since 5 years in New Delhi. Now I have upgraded to 4g sim. I have no problem with airtel sim regarding network coverage But when it comes to low balance issue It to s pathetic. If you are on low balance then whenever you call The operator gives you a lecture of 5 minutes in various languages Like English hindi and one regional language. If you want an urgent call they will dislocate your mind by their logic less speech.
The customer support is good and yes the service center are easily available. But yes they charges a high price for their stuff.