Here are my experiences with Exatt Technologies.
I have been using their 512kbps plan for almost 1 year now. The download cap is 60GB per annum. I paid approx. 16,000/- for my 1 year subscription, which is EXCELLENT value for money. The maximum sustained download speeds I achieve are around 50KB/sec, which translates to approx 400kbps. I have NEVER seen a download exceed 55KB/sec, so their 512kbps plan has NEVER achieved 512kbps.
They have 24x7 tech support. The support staff are courteous, but often suggest frustratingly stupid things to the user, which may be of help when a novice user has a problem, but is very annoying when you are well versed in PC hardware and networking.