I am very old customer of airtel broadband. I have been using it for 6 years (since 2005). Well I must say one thing, they have a very good customer service. Lately though, they have disappointed me in few occasions, but till 2010, it was problem free. Airtel has upgraded connection speed of all customers in 2009 free of cost. My connection was for 256KBPS, and it had been upgraded to 2 MBPS without any extra charge.
However, lately, they have increased the rental cost by Rs 100 without informing. Customer service guys says, I was informed via email which I never received. I was very irritated, but just gave up fighting. But anyway, overall I am very happy about the service. Hope they reduce the cost, and go back to the old price.