I have tried all 4 operators who have 3G - Idea, Vodafone,
Tata and BSNL
I can say IDEA worked for me for most of places with good speed.
Vodafone has good speed but it does not have coverage at my home, so can't use it.
Tata has very good speed is some places, but overall coverage is bad.
BSNL never worked with 3G while I tried at home as well as outside.
Idea and Vodafone plans are costly as well and almost identical.
Tata and BSNL are cheaper but not available widely.
Besides above 4, my colleage uses Airtel 3G, which perhaps roams 3G on Idea/Vodafone, and he also gets good speed, but plans are costly.