I have traveled to India for the last four years as a volunteer at a school for street children, and I can answer that while India's tourist destinations are great, this is also a wonderful destination for service vacations.
I volunteer as a computer and English instructor at Nav Jeevan Sanstha in Nagpur, India. This is a free school for slum kids and we are attempting to improve education there by using free Internet teaching aids such as Khan Academy. This school is poor, so we get to try educational approaches that would not be possible elsewhere. Our only gauge of success is improved test scores and better performance in class.
One of the big perks of a service vacation is living in and working directly with people from another culture. In addition to the satisfaction of improving the lives of children who have no other chance at success, I also get to live in direct contact with the India that is never apparent from tourist hotels. We eat the local food, travel on the local buses and auto rickshaws, shop in the local bazaars and talk to local residents.