Airtel 512kbps unlimited plan for Rs. 1999 per month.

I just noticed that airtel has new plan and that is Rs. 1999 for 512kbps unlimited data transfer.
Considering Rs. 999 for 256 kbps ul I think it should be around Rs. 1600 per month.

On their site they mention as Speed upto 512kbps, what that mean? Means that there are variation and its not really 512kbps all the time. Its clear that it is shared.
Can anyone give me idea how are the speed of their airtel connection, speed variation throughout the day (no matter whatever plan they have).

On bsnl , which I am currently using, they are are contant, sometimes appears as if I have dedicated 256kbps connection rather all the time, if it is same for 512 kbps airtel then I will love to switch to it.

I do not have Airtel !