I am a satisfied customer of Airtel Broadband. Usually people post reviews only when they have a complaint. I have posted this review just to give a positive rating to my favourite ISP. I highly recommend it.
The connection is always up and ready to serve you. Rarely the service gets disrupted for maintenance activities. But such activities are scheduled late at night to reduce the impact on the customers. Moreover the customers are informed beforehand through SMS so that they can plan accordingly.
The only drawback I can think of is the installation procedure. It involves some drilling and masonry. A connection box is fitted on the wall to which the telephone and modem are connected. This may not be possible for those living in rented buildings - if the owners do not permit.
P.S.: I have been using Airtel Broadband(256 kbps unlimited scheme) for more than 1.5 years.