If you are looking for a reliable internet connection then after Airtel, ABC Broadband is good in my experience. The worst is BSNL. I live in Noida and we do not have Airtel in my apartment.
The only problem is that I have a 4 mbps plan I do not get that speed on public holidays and Sundays. The speed is as low as 56 kbps in the secondary policy which is a joke.Customer care is dumb. All they know is to follow a troubleshooting flowchart. In any case I dont think that they can resolve the issue since it seems that ABC Broadband has sold way too many connections and at peak load they do not have the capacity to provide the entire user base with the promised speed. This really should be a criminal offence, alas in India you can get away with anything.
Spectranet is another provider, they provide excellent speed as promised at most times. The only problem is that it works on a physical cable like cable TV and there are lots of faults and frequent downtime. I use this as my Primary Connection with ABC Broadband as a back up. On days that I do not have Spectranet and it is a public hoiday then I use my Tata Photon, this offers 600 kbps of speed at my home and is better than ABC Broadband on a holiday.