I have been using tata docomo for the past 2 years.It has its advantages and disadvantages.
First coming to the pros of using a docomo sim:
You get very good plan for 6 months and the also the data pack and talktime offers are pretty cheap compared to othe operators like airtel, vodafone etc.
you get decent coverage in cities and other places
Cons of using a docomo sim:
COVERAGE!the coverage sucks compared to other operators.I have been to many places where the coverage for my freinds phones who mostly use airtel is full and I on the other hand get hardly any coverage.
But still very satisfied with the operator.