I have using Tata Docomo Sim but it not looks so good .Sometimes it works properly but sometimes there is full of network but useless. If somebody calling you it will say like the person you are calling is busy or out of service.However you are neither busy nor out of service.
There is another thing about Tata Docomo that it does not have 3G network yet and 2G is not enough for today. In this situation Reliance is better than Tata Docomo. Hence My opinion is Tata Docomo is not as usefull as other network like Reliance.