I have used Airtel, Idea, Vodafone and Aircel.In my opinion, Vodafone is the best among all. Its coverage area is large , so one can have access to network and 3G services almost anywhere.
The customer service is average, but atleast better than others. I think that charges are bit higher as compared to idea.I did not like any of their schemes whatsoever.
I am using Vodafone since an year and I am satisfied to a great extent.