I am using Tata Docomo sim from past 2-3 years and I am not so happy from the mobile operator. The network coverage is good in the city but network coverage in Villages or rural areas is very bad. Even in some area within city the network goes and you will get roaming network on your phone very quickly.
The net speed is also not good as compared to other providers in the same market segment. In terms of charging or billing, it is not so costly, And it also have agood customer support.
Overall Tata docomo is quite decent for the people living in the urban areas, but not so good for the people residing near rural areas.