I too have maintained an Airtel prepaid sim for the last 5~6 years, using it for around a week every 3 ~ 6 months. Put money into it every trip (just usage - haven't seen any value reduction otherwise). Other than early issues in activating and deactivating data service (back when data was Rs. 20 per day; ended up having to show up at Airtel's local office to have it resolved a few times) have not had any issues.