I myself am using airtel, and I have to say that it is the speedest network that, i have used in india.
But the coverage is poor.
Most of the time I lose my connection.
The customer service is okay. But it doesnt has any coverage at all.
The charges and billing are almost fair enough.
The signal strength becomes only weak when i, start using my data connection.
The network towers are weak and are most of the time unavailable.
The conclusion is this network is good for calling not for using data.