I am using Airtel Broadband unlimited 3000 plan at home and the 512 kb/s commercial plan in my office for the last past 5 years and have been satisfied .There have been a few hiccups specially in the commercial plan sometimes (only 3 times in five years) but I would still recommend it over any other company atleast here in Delhi NCR region.
Here are the good things about airtel broadband:
The service and support staff are super quick when compared to the snail behavior of tata , mtnl etc
The connection is never disconnected .
The speeds are constant.
The bad things are :
1 Sometimes they automatically charge you for airtel pc secure by activating it automatically but you can deactivate it , just be careful when you go through your bill.
2.Speeds sometimes drop once in two months for a few hours for some odd reason.