I am using Airtel for almost 17 year from now. In beginning, it was the best available service provider then, and with time, it lost all its emphasis and attention on customer.
Speed of ISP: Speed is good at certain GB of data and then it comes down drastically. Airtel gives very less GB for high speed data in comparison to many other not-so-branded but good players in the market. The price that Airtel offers for 25-27 GB data, the same is now available for more than 80 GBs by other players.
Connection Reliability: Connectivity issues are now very common and more common is the bad experience with their SLAs. Their commitment never get fulfilled and connectivity issues with speed issues persists. However the call center executives will say they can see no-issue in your area, but you being in that area and facing the issue, starts getting frustrated with their negligence towards the issue. This gets more prominent with their 4G coming in place.
Service and Support: Not so good. Issues gets pending for long and no ear comes to listen from Airtel.
Airtel is just loosing the brand value and many people are moving on with other market players. Many just sticking to Airtel for free and unlimited Local and National moonthly calls on the payment of Rs 100; else probably the number people moving on would be very high.
Not recommended for new users. Try other service providers.