I stay in Mumbai and have used Vodafone, Airtel and Dolphin during last 10 years.
It is a fact that the best cellular operator in Mumbai is Vodafone. Airtel in Mumbai is the worst.
I am using Dolphin for last 6 months. As most other reviews here suggest, the network coverage is bad. But purely from an economic point of view, Dolphin is THE cheapest choice. You get some benefits like low SMS charges, per second billing etc. free which you have to pay (using bonus coupons) for other operators.
Pricing: As cheap as it gets. Bonus talktime is given freely on prepaid recharges even as low as Rs. 125. I once used their SMS coupon (something like 500 SMSs free - any network, local/STD - for Rs. 30, valid 30 days), and it is now well past 2 months but still I am not charged for any SMS that I send :-P
Also, I am a heavy GPRS user, and the GPRS charges of 1 paisa per 10 KB (without any bonus coupons) are way too cheaper than others. Rating: 9/10
Network Quality: The voice and data transfer quality of Dolphin, if it connects, is second to none. The key word here is if. The network is good outdoors, but it gives many problems indoors. I really do not know why they can’t improve the coverage. Because their network equipment seems to be of good quality - if it connects the quality is really good. With a better network coverage and marketing, I am sure Dolphin will give a tough competition to any other player with its cheapest tariffs. But be ready to dial a number at least 2 times on average to make sure your call gets through :D
Rating: 6/10
Customer Support: Because the user base in small, the call to the customer care gets connected without waiting on the line for even one second. And unlike most other private operators, the calls to CC are free. The CC people are average to dumb, but get the work done if you spoonfeed them about your query. Rating: 7/10
Conclusion: This is a good choice as a second number (as most people have 2 connections these days in metros). Don’t expect the network reliability of Vodafone or high end VAS like BlackBerry service, M-Shop and Pocket Apps etc. It will greatly reduce your monthly mobile expenses. Overall Rating: 6/10