I post my experience with Airtel, Maharashtra (Pune). I availed their pre-paid connection for about a year. Later, I switched to a post-paid connection on Idea network. So this review is mostly a comparison of the two network providers.
The Airtel network is pretty good in terms of the signal strength and the overall coverage area. Ive never had problems in most places in and around Pune. But, I never got a signal within my apartment on the first floor; which is in a fairly prime locality in Pune. Today, on the Idea network, I get a pretty good signal within my apartment. Of course, this could be the outcome several factors (distance to nearest cellular tower etc) and others subscribers could have experienced the contrary.
Airtel has been advertising a lot about their congestion free network on the billboards of Pune. Well, I did not find much of a difference in the network availability of both Airtel and Idea. So, all this noise about congestion-free network is no big a deal. In this era of tough competition, no service provider can get away with a poor QoS (Quality of Service). Esp with sites like MS around :)
The number of retail outlets which stock Idea recharge vouchers are much higer than those which stock Airtel recharge vouchers. Sometimes you have to hunt for a while to buy an Airtel recharge voucher.
One thing I really did NOT like about Airtel, is their customer service. Ive had a very bad experience, trying to get connected to their customer service on the phone. What I really wanted was to convert my pre-paid connection to a post-paid one. Most of the times the phone at the customer service was ringing without any reply. When I did get a reply from a customer service exec, she did not help me much. After several attempts, I got pretty bugged of and decided to switch to another service provider.
Well, I got an excellent service from Idea Cellular. Their customer service guy actually dropped by my office and gave me a tempting corporate offer on post-paid connection. Airtel lost me, as a customer, merely because of their poor customer service. Bottomline, the Airtel network is decent, but far from being good.