I am using it from 3 years and I personally have no issue with airtel. Except that their plans are a bit costly(their friends and family and sms plans are not worth for money).
GPRS speeds really s*ck on my class 32 EGPRS device (Nothing more than 40kbps thought they have EDGE enabled networks). The customer care people are very responsive. (Personally except in one case I got all my problems solved within time and they are very polite.)
I am fearing a lot how 3g plans are going to blast.( I am planning to switch to BSNL permanently after MNP comes in AP).