I have been having Airtel broadband for more than 6 months now (256 Kbps
with 1 Gb limit/mo at Rs. 599/month with free Telephone number).
The main thing I have to say is that their service is quite reliable.
This was a big relief after trying out Tata Indicom Broadband which
though zipping fast (512 kbps meant atleast 250+ actual speed) was
practically useless because it would break every 10 minutes in the
middle of a big download or a VPN connection.
Airtel on the other hand maintains its connectivity quite well. I would
recommend an Ethernet card with an Ethernet modem as it involves no
driver installation on your system to slow it down. This is the proper
way to do it but they might try to give a USB modem with a driver CD as
it is cheap.
The only downside I found is that sometimes during peak hours the speed
drops close to a dialup like 75 Kbps. But mostly the speed is above
120 Kbps and touches 160+ when it is fast.