I am airtel customer from since 5 months.if we talk about speed the it is very good but when once your extend your quota(ie. limit in gb) the airtel gives you internet @ speed of 512kb/s. the problem is that from this 512kb/s many times you receive not more than 150 or 200.
but the customer support is excellent. whenever you have problem just drop mail to them it will fixed soon by airtel.
The main issue is that the cost is extremely high when compared to the competition.