Unlike the US or Europe where people actively use online mapping websites to plan their road trips, the map culture has not arrived in India yet but the situation may change very soon with Google entering the road mapping business in India.
Googler Krishna writes that road data for lot of Indian Cities and even towns are now available on Google Maps. You can also use the Google Map data to plan trips between cities as there's detailed data for the Indian national highways as well.
Will Google Maps replace MapMyIndia ? The immediate answer is in the negative since only MapMyIndia.com provides detailed street level information of Indian Cities as of now. Google Maps can however fill in the gaps like satellite views and hybrid maps.
Some prominent Indian cities on Google Maps - New Delhi, Indian Silicon Valley, City of Taj Mahal, Hyderabad, Chennai, Bombay (Mumbai) and Calcutta.
Related: India Road Maps with Driving Directions